When used with PC floppy controllers, there is no practical reason to use a dual-speed 5,25" drive. The PC BIOS expects a 360rpm-only drive if you configure the drive to "1.2MB". This only works because PC-type floppy controller cards support the uncommon data rate of 300kbps instead of the common 250kbps, which results from rotating DD disks "too fast". In non-PC systems that do not support 300kbps, the dual-speed mode might be required. The primary issue with writing 360K disks an an HD drive is indeed the track width and not the rotational speed. This is technically not related to the distinction between HD (500kbps, 15 sectors / track) and DD (250/300knps, 9 sectors / track), but instead related to the track count / track spacing, which is 48tpi / 40 tracks on DD drives and 96tpi / 80 tracks on HD drives.

Just as with CD drives, there might be an advantage for signal integrity / reliability when using a lower speed, but the speed difference between 300 and 360 rpm is low enough that the practical advantage of using a lower speed is likely non-existing in practice.

mkarcher wrote on 2023-11-15, 06:40:

When used with PC floppy controllers, there is no practical reason to use a dual-speed 5,25" drive. The PC BIOS expects a 360rpm-only drive if you configure the drive to "1.2MB". This only works because PC-type floppy controller cards support the uncommon data rate of 300kbps instead of the common 250kbps, which results from rotating DD disks "too fast". In non-PC systems that do not support 300kbps, the dual-speed mode might be required. The primary issue with writing 360K disks an an HD drive is indeed the track width and not the rotational speed. This is technically not related to the distinction between HD (500kbps, 15 sectors / track) and DD (250/300knps, 9 sectors / track), but instead related to the track count / track spacing, which is 48tpi / 40 tracks on DD drives and 96tpi / 80 tracks on HD drives.

Just as with CD drives, there might be an advantage for signal integrity / reliability when using a lower speed, but the speed difference between 300 and 360 rpm is low enough that the practical advantage of using a lower speed is likely non-existing in practice.

Thank you for this explanation. I honestly didn't know any of this, and it sounds like as long as the drive is in a PC then I can just disable this. The machine definitely is doing weird things with one disabled floppy controller and the original one added. For example it will read DD disks fine, but you put a HD disk in it will let you switch to the drive but it shows file not found with a DIR command when there is files on the disks. It does this with the 3 different IO cards I had. If I remove the IDE controller with disabled floppy controller and boot from a floppy it works as expected for DD and HD in the speed sense mode. But I am gonna forget this and go back to just one I/O Controller hard set the floppy to HD mode and call it good. Thanks..