Most 5"1/4 1.2MB floppy disk drive can't properly write to 360KB disks. You need a 360KB 5"1/4 disk drive to be able to write to 360KB disks.

Back then I liked the Teac FD-55[G]F because it did that quite well.

Wait, please tell, do you use a 360KB or 1.2MB floppy disk in your 1.2MB drive ?
Formatting to whatever capacity should work, as long as the medium matches the floppy drive type.

Also, please say which version of DOS you are using.
Different versions of DOS came with different versions of Format (switches may differ).

Edit: On MS-DOS 6.x, FORMAT X: /F:360 formats a diskette to 360KB. FORMAT X: /4 is meant for real 360 diskettes only.
More information available here: https://www.vfrazee.com/ms-dos/6.22/help/format.htm

You also can use
FORMAT B: /N:9 /T:40

hi - more info:

- floppy drive is 1.2 5.25
- diskettes are HD
- dos 6.22

according what i read above (thx guys) it looks like i can not use /4 option because i have hd diskettes

i will try the other option you mentioned

btw - why i am doing this:

i found an old 5.25 360kb floppy drive
i want to test it
it is seen by bios correctly
dos is trying to change to b: and led is on when i enter b:
but so far i have test not finished -until i r/w a diskette

tell me pls if the hd diskette force formatted to 360 should be readable in360 floppy or i need original dd diskette

A HD diskette force formatted to 360K by a 1.2MB drive is unreadable by a 360K drive.

There's a big difference between HD and DD diskettes: the texture of the diskette data itself is different between the two. I had a hard time formatting 720K DD diskettes in my 1.44MB HD diskette drive as the format command thought the diskette that was inserted was a 1.44MB diskette. After typing in:

1format A: /f:720

I'm able to read and write to the 720KB diskettes. The 5.25" diskette drives don't like reading and writing to 360KB diskette between the 360KB and 1.2MB drives in some cases or in all cases, including formatting 1.2MB diskettes as 360KB (doesn't work like that). Much like formatting a 1.44MB diskette as a 720KB diskette, or using a 720KB diskette as a 1.44MB diskette (impossible to do that).

AFAIR unless the drive is HD only (and it could be - especially the 3.5" ones that were manufactured just as floppies were all but phased out) it should be possible to format both to HD and SD/DD. But on drives that auto-detect the format it might not be possible to force it via software, but then again all it takes is covering the cutout/hole to fool the detection.
The drive that formatted the floppy should read/write it just fine. It might not work on a real low-density drive though as the magnetized areas will be too thin and out of alignment with the head. It's a bit of lottery. A HD drive simply skips every other track and what's "left" in there will interefere with the read on low density one.

The only way I've ever been able to properly format a 360KB floppy in a 1.2MB drive was by scrambling the entire disk first by running a strong magnet (I use one removed from a hard drive) across the entire surface of the disk. This way there's no leftover data from the previous contents to cause read errors when you then try to read back the disk in a 360KB drive. This has worked well for me, but it does have the downside in that if you want to change any of the files on the disk, you have to copy the current files off, redo the magnet wipe & format, then re-write the entire disk. This can get tedious, so I only use this method for disks I don't plan on changing much, if at all.

However, I will note that I have never used this method to try to convert 1.2MB disks into 360KB disks, so I don't know if it will work in that case. I use it only to format actual 360KB disks in a 1.2MB drive.