Walt.Gregg.Juneau.AK.US (Nov. 17, 2020)

Reading Legacy 5-1/4 Inch Floppy Disks

Why care about being able to read legacy 5.25 inch floppy diskettes? Sometimes, one of them will contain an irreplaceable treasure. It might be the forgotten words of a poem or song you wrote. I wanted to be able to check before throwing anything away. But Intel removed motherboard support for 5-1/4 inch floppies in 1998 and 3-1/2 inch floppies in 2002, though PCs were still being sold with them after that.

I arranged to overrule Intel. From descendents of IBM OS/2, this is completely transparent (ArcaOS and EComStation 2020). I can simply address the 3-1/2 inch drive as 'A:' and the 5-1/4 inch drive as 'B:', despite Intel having dropping dual drive support! And with the assistance of Sydex 22disk, which you can still register, I find I can still read floppy disks created with the CP/M Kaypro 2 as long ago as 1983 (Sydex 2020). Here's how I did it:


From OS/2, drive A now accesses 3-1/2 inch 720 and 1440 kilobyte disks, while drive B accesses 5-1/4 inch 360 and 1200 kilobyte disks.

Trap: If you write to 5-1/4 inch disks holding data written on 360 kilobyte drives the disk may be damaged beyond recovery. I've seen this result in readback with intermixed contents of old and new data. The problem is that the 1200 kilobyte drive only overwrites a narrow strip of any track previously written by a 360 kilobyte drive. Workaround: Protect 5-1/4 inch disks holding data written on 360 kilobyte drives by putting black electrical tape over the write protect notch. Don't reuse them without bulk tape erasing them before reformatting them. The RoBINS Magnetic Bulk Tape Eraser TM-88 (R24-015-922) currently runs around $30 used.

To format disks for MS-DOS:

format a: [/f:1.44)
Format a 3-1/2 inch high density disk (1.44 megabytes). The /f:1.44 is not required.
format a: /f:720
Format a 3-1/2 inch double density disk (720 kilobytes). The /f:720 is required.
format b: [/f:1.2]
Format a 5-1/4 inch high density disk (1.2 megabytes). The disk must be a high density disk. The /f:1.2 is not required.
format b: /4
Format a 5-1/4 inch double density disk (360 kilobytes) in the 1.2 megabyte drive. Per the warning above, if the disk holds data written by a 360 kilobyte drive, bulk tape erase it first. The /4 is required.

CP/M access

To work with disks in CP/M formats, I obtained the Sydex shareware 22Disk. I installed it in OS/2-DOS. With the OS/2-DOS settings HW_ROM_TO_RAM=ON, HW_TIMER=ON, and IDLE_SENSITITY=100, it even works in a window.

Trap. You can't read single density disks such as the Xerox 810 single sided single density format often used to distribute bundled CP/M Kaypro II software. Reportedly, after the PC switched from the then conventional CP/M WD FD1793 controller to the NEC µPD765 controller, you could still read single density disks by covering the index hole with black tape. But the AT switched to the Intel 8272A chip, and the PS/2 to the Intel 82072A chip, and single density support vanished. Workarounds: use a CP/M machine to copy the disks to a double density format, or use a conversion service.

In May 2018, having confirmed that 22disk works and works well, I emailed info [at] sydex.com to inquire about registration. They sent instructions for making a $25 payment via email using paypal. After payment they promptly sent me a download link with the latest full registered version.


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