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Read DD floppy in HD 5.25" drive

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First post, by jhandsel

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I got an old high density 5.25" floppy drive (Mitsubishi MF504C), cleaned it up, and hey presto it reads high density floppies (1.2MB) without a problem under MS-DOS 6.22.

However... I haven't had any luck getting it to read double density (360KB) floppies. I thought the high density drives could read double density disks?

Is there something I need to do to tell DOS which type of disk I'm trying to read? Or is there something I'm missing?

Reply 1 of 3, by mkarcher

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5.25" disk drives have more compatibility problems with DD media in HD drives than 3,5" drives, because 5.25" DD media (in PC usage) has only 40 wide tracks and HD media has 80 narrow tracks. The read/write head in HD drives is made for the narrow tracks, so it can't properly write the wide tracks. On reading, it reads just the center of the wide tracks, which should work fine if the track is not severly damaged. So, reading is expected to work.

It is very important though, that the BIOS knows the drive is a 1.2M drive. If the CMOS is set for a 1.44M drive, you would obtain exactly the problem you describe: HD works, but DD fails. DOS automatically gets the drive type from the BIOS, so no need to tell anything to DOS.

Reply 3 of 3, by Horun

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Glad you figured it out ! Yeah single sided DD would make it a 160k or 180k disk (depending on 8 or 9 sectors/track) which most 1.2Mb cannot read.

Hate posting a reply and then have to edit it because it made no sense 😁 First computer was an IBM 3270 workstation with CGA monitor. Stuff: https://archive.org/details/@horun

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