apologies sorry I didn't answer the other questions
I've taken some photos of the actual drive hope this helps

https://www.manualslib.com/manual/1797625/Mit … l?page=4#manual

The Mitsubishi M4851 is a high-performance, double-sided, double bit and double track density flexible disk drive

That is a very old DS/DD drive. So you need to select 360kB in BIOS.

Be aware that most 5.25" floppy disks you might find will be 1.2MB, so this will only be useful for XT and the oldest AT era stuff. A DS/DD drive can't read 1.2MB DS/HD disks.

Damn, my eyes really are going. It's an M4854, not M4851, so DS/HD after all.

very interesting thanks for letting me know will select that in bios and try and get some 360 kb disks to use

jude1977 wrote on 2025-05-16, 03:08:

very interesting thanks for letting me know will select that in bios and try and get some 360 kb disks to use

You can use 1.2Mb disks but they will have to be reformatted to 360k. Once you do that they generally can no longer be properly changed back to 1.2Mb disks (unless they are fully bulk erased)
So if you have a few blank 1.2Mb go ahead and try those....be sure and label as 360k.

dionb wrote on 2025-05-15, 20:47:

https://www.manualslib.com/manual/1797625/Mit … l?page=4#manual […]
Show full quote

https://www.manualslib.com/manual/1797625/Mit … l?page=4#manual

The Mitsubishi M4851 is a high-performance, double-sided, double bit and double track density flexible disk drive

That is a very old DS/DD drive. So you need to select 360kB in BIOS.

Be aware that most 5.25" floppy disks you might find will be 1.2MB, so this will only be useful for XT and the oldest AT era stuff. A DS/DD drive can't read 1.2MB DS/HD disks.

OP's drive is an M4854, not an M4851.
https://retrocmp.de/fdd/mitsubi/m4854_i.htm

That would make it a 1.2M high density drive, AFAIU.

Horun wrote on 2025-05-16, 04:02:

jude1977 wrote on 2025-05-16, 03:08:

very interesting thanks for letting me know will select that in bios and try and get some 360 kb disks to use

You can use 1.2Mb disks but they will have to be reformatted to 360k. Once you do that they generally can no longer be properly changed back to 1.2Mb disks (unless they are fully bulk erased)
So if you have a few blank 1.2Mb go ahead and try those....be sure and label as 360k.

DS/HD disks cannot be used in common DS/DD drives because of another ferromagnetic layer. I remember that exactly for last 35 years ))

Horun wrote on 2025-05-16, 04:02:

You can use 1.2Mb disks but they will have to be reformatted to 360k. Once you do that they generally can no longer be properly changed back to 1.2Mb disks (unless they are fully bulk erased)
So if you have a few blank 1.2Mb go ahead and try those....be sure and label as 360k.

Not guaranteed ... sometimes seems to works in some drives, but not at all or unreliable in others.

HD has a different flux density then DD and uses different media!

DaveDDS wrote on 2025-05-16, 16:55:

Horun wrote on 2025-05-16, 04:02:

You can use 1.2Mb disks but they will have to be reformatted to 360k. Once you do that they generally can no longer be properly changed back to 1.2Mb disks (unless they are fully bulk erased)
So if you have a few blank 1.2Mb go ahead and try those....be sure and label as 360k.

Not guaranteed ... sometimes seems to works in some drives, but not at all or unreliable in others.

HD has a different flux density then DD and uses different media!

Agree it is not guaranteed or reliable over long usage if it does work. Did not have much issue doing it with a Teac FD55-BR and a few 3M disks a few years ago but that could have just been luck 😀
I should have said "you could try"

Horun wrote on 2025-05-16, 21:22:

DaveDDS wrote on 2025-05-16, 16:55:

Horun wrote on 2025-05-16, 04:02:

You can use 1.2Mb disks but they will have to be reformatted to 360k. Once you do that they generally can no longer be properly changed back to 1.2Mb disks (unless they are fully bulk erased)
So if you have a few blank 1.2Mb go ahead and try those....be sure and label as 360k.

Not guaranteed ... sometimes seems to works in some drives, but not at all or unreliable in others.

HD has a different flux density then DD and uses different media!

Agree it is not guaranteed or reliable over long usage if it does work. Did not have much issue doing it with a Teac FD55-BR and a few 3M disks a few years ago but that could have just been luck 😀
I should have said "you could try"

Either way, the drive in the photo OP shared is an M4854, NOT an M4851.

That should be an an HD drive, NOT a DD one.

darry wrote on 2025-05-17, 03:52:

Horun wrote on 2025-05-16, 21:22:

DaveDDS wrote on 2025-05-16, 16:55:

Not guaranteed ... sometimes seems to works in some drives, but not at all or unreliable in others.

HD has a different flux density then DD and uses different media!

Agree it is not guaranteed or reliable over long usage if it does work. Did not have much issue doing it with a Teac FD55-BR and a few 3M disks a few years ago but that could have just been luck 😀
I should have said "you could try"

Either way, the drive in the photo OP shared is an M4854, NOT an M4851.

That should be an an HD drive, NOT a DD one.

Yes sir, I saw you correctly ID that info earlier. Was just posting a response to the ability to use 1.2mb disks in a 360k drive based on my limited experience doing it.

Horun wrote on 2025-05-17, 05:31:

darry wrote on 2025-05-17, 03:52:

Horun wrote on 2025-05-16, 21:22:

Agree it is not guaranteed or reliable over long usage if it does work. Did not have much issue doing it with a Teac FD55-BR and a few 3M disks a few years ago but that could have just been luck 😀
I should have said "you could try"

Either way, the drive in the photo OP shared is an M4854, NOT an M4851.

That should be an an HD drive, NOT a DD one.

Yes sir, I saw you correctly ID that info earlier. Was just posting a response to the ability to use 1.2mb disks in a 360k drive based on my limited experience doing it.

Ack, sorry if that came out the wrong way. I had not noticed that anyone had taken note of that yet when I responded.

I have never really owned a 360K drive (at least not very long). It would be fun to experiment with one at some point. I wonder if some 360K drive designs might indeed have had better luck with HD media than others, as you suggest, possibly even to the point of long term reliability.

thanks for all your replies so I'm a bit confused is my floppy drive that i showed in the picture is it hd or dd not Shure.

if it was dd would i set it 360kb setting or 1.2mb setting in bios

jude1977 wrote on 2025-05-17, 06:19:

if it was dd would i set it 360kb setting or 1.2mb setting in bios

If it was DD it would be set as 360K in BIOS.
Now that it's HD, set it as 1.2M.

General advice: when you are troubleshooting, try with the minimum possible configuration to eliminate other factors. In your case that would be to remove any other floppy drives connected like the 3.5" you also have, boot in pure DOS and try there, connect the drive to another PC to try it there. Unless you do any of these that have been suggested by multiple users before, there is really no point analyzing any further.

jude1977 wrote on 2025-05-17, 06:15:

thanks for all your replies so I'm a bit confused is my floppy drive that i showed in the picture is it hd or dd not Shure.

darry wrote on 2025-05-16, 04:46:

OP's drive is an M4854, not an M4851.
https://retrocmp.de/fdd/mitsubi/m4854_i.htm

That would make it a 1.2M high density drive, AFAIU.

jude1977 wrote on 2025-05-17, 06:19:

if it was dd would i set it 360kb setting or 1.2mb setting in bios

If it was DD you would set it to 360K.
Since it is a HD, you set it to 1.2MB.

Though, I must admit, I'm a bit confused by that 77 tracks statement/limitation.

The M4855 is the successor of the M4854 (both HD), which is a pure 1.2 MB drive with 77 tracks. The M4855 has 80 tracks. The M4854 can be used instead of an 8 inch drive because it has the same parameters.

weedeewee wrote on 2025-05-17, 07:51:

...Though, I must admit, I'm a bit confused by that 77 tracks statement/limitation.

The M4855 is the successor of the M4854 (both HD), which is a pure 1.2 MB drive with 77 tracks. The M4855 has 80 tracks. The M4854 can be used instead of an 8 inch drive because it has the same parameters.

I'm not familiar with that exact drive, but references to 77 track is entirely possible/understandable.

1.2M 5.25" HD drives are electrically much more similar to 8" drives than 5.25" DD drives (HD/8" =360rpm 500kbps data rate,
DD = 300rpm 250kbps) - 8" drives were never officially used in the PC (see my ImageDisk docs for details on making a cable to
allow connecting one), but were common in the early days, pre-existing to 5.25" drives. The main difference an OS would see is
that an HD drive has 80 tracks, but an 8" drive has only 77 tracks (there's also a "TG43 signal which isn't used on 5.25"s).

Which bring an interesting question....

It's not uncommon to use a 1.2m HD 80 track drive as a replacement for an 8" drives ... only 77 tracks of the media get used,
but everything works.

Does a M4854 have physically 77 tracks over the media? Ie: is it different track spacing?
I could see this being a thing during the time before true 80-track HD drives existed...

Or does it just have the inner "stop" in a different position.
The former would means media written on the M4854 would only be readable on it. The latter would mean either drive could
read the media.

An unrelated but somewhat relevant quirk of the original AT is that it still used the Nec 765 FDC - which was designed era of
8" drives, IIRC it's "Home head" function fails if it does not see the track-zero input after 77 steps.

I ran into this while developing ImageDisk which talks directly to the FDC without going through BIOS.
After reading/writing an 80-track disks the head would be at track-79 and accessing another disk would fail
to "home" - I had to make it try homing the head twice!

This is also the reason IMD supports 256 tracks/sectors - I didn't expect such media would ever actually exist
but I didn't want to put artificial limitations on what the .IMD format could represent.

jude1977 wrote on 2025-05-17, 06:15:

thanks for all your replies so I'm a bit confused is my floppy drive that i showed in the picture is it hd or dd not Shure.

I'm sorry, I started that confusion as I misread the number on the drive sticker. It is HD after all.

hi all I've done a bit more testing I've tried another 5.25 floppy disk drive on exactly the same cord and drive witch I've got set to drive A and that floppy disk drive works fine and when I try the floppy drive that I've taken the photos
of on the same ribbon cord and drive A it still doesn't work both on 3.15 and 1.2 mb setting in bios I've got the floppy drive that doesn't work out of the machine running I can see the disk turning and laser head going
back and forth a little bit so I'm pretty Shure the ribbon cable and having drive A selected is ok so I'm pretty Shure it must be something to do with the actual floppy drive.
I've brought a floppy disk drive cleaner disk on eBay so when I get that ill give that a try just in case the heads are dirty.

jude1977 wrote on 2025-05-17, 12:59:

hi all I've done a bit more testing I've tried another 5.25 floppy disk drive on exactly the same cord and drive witch I've got […]
Show full quote

hi all I've done a bit more testing I've tried another 5.25 floppy disk drive on exactly the same cord and drive witch I've got set to drive A and that floppy disk drive works fine and when I try the floppy drive that I've taken the photos
of on the same ribbon cord and drive A it still doesn't work both on 3.15 and 1.2 mb setting in bios I've got the floppy drive that doesn't work out of the machine running I can see the disk turning and laser head going
back and forth a little bit so I'm pretty Shure the ribbon cable and having drive A selected is ok so I'm pretty Shure it must be something to do with the actual floppy drive.
I've brought a floppy disk drive cleaner disk on eBay so when I get that ill give that a try just in case the heads are dirty.

Which type is the working one (HD or DD)?

Sure way to tell is to grab my ImageDisk, all you really need is IMD.COM (and IMD.HLP if you want the interactive
help to work) which will fit on a floppy. Do this on the working drive A: with the non-working drive installed as B:

In IMD, under "S)ettings" select drive A:, then under "A)lignment/test", press '4' to seek to track 40.
If B: is HD (80 track) the head will move to about 1/2 way along the access oval hole in the diskettet.
If B: is DD (40 track) it will move to nearly the inside end (and may bump the stop since the drive technically
has only tracks 0-39 - most allow and extra one or two).

Repeat, selecting drive B:
This will tell you if they are the same type, and what the BIOS should be set to.

When recovering a "dead" floppy drive, first thing I usually recommend is try cleaning.
If using a cleaning disk, just accessing the drive to make the disk spin against the heads will help a bit,
but you really want more movement. IMD has a pretty good "C)lean head" function which not only
spins the cleaning diskette, it also "scrubs" the heads back and a forth through the possible
cylinders.

hi its the HD thats not working and the DD is working thanks for the link i will try the IMD.COM program.