Just adding a density notch to a 720k disk that has been formatted as 1.44mb should make it readable using a SuperCard Pro or similar device. It is easy enough to cover the new notch back up if needed.
In normal 1.44mb drives, the density notch controls the analog filter the drive uses to create the digital data out signal. In most 1.44mb drives a high density (500kbps) signal filtered as low density (250kbps) will get partially filtered out as "noise" and therefore will be unreadable.
Almost all 1.44mb floppy drives IGNORE the density line signal sent by the SupoerCard Pro, similar flux device, or floppy disk controller. These drives exclusively rely on the density notch to determine which density filter to use.
I have only one 1.44mb drive - a Sony MPF520-D P/N 04H7405 FRU 93F2361 - that does indeed respect the density line, thereby allowing a SCP or floppy controller to override a density notch. (Note: some drives use the density line to engage the 360RPM Japanese "Mode 3") I think that drive was used in IBM Valuepoint systems in order to maintain compatibility with these disks. It is otherwise a perfectly standard 1.44mb drive.
It is likely that some 720k disks may not have been capable of holding a high density signal, and have degraded over time. These will not be readable in any drive, no matter what. The magnetic formula used to make the disk cookies differed between manufacturers. That said, I have many 720k disks that were intentionally notched, successfully used as high density, and are still readable today.
I don't have any sample disks like this off hand, so I can't check - but It is also conceivable that some PS/2 systems may have written something differently on the boot sector or file system that may have prevented *DOS* from reading them properly. But in that case, a flux level reader, once the drive's density filter is set properly, should still be able to read the content. Make a dump, feed it to HxC and the sectors should light up green.
In normal 1.44mb drives, the density notch controls the analog filter the drive uses to create the digital data out signal. In most 1.44mb drives a high density (500kbps) signal filtered as low density (250kbps) will get partially filtered out as "noise" and therefore will be unreadable.
Almost all 1.44mb floppy drives IGNORE the density line signal sent by the SupoerCard Pro, similar flux device, or floppy disk controller. These drives exclusively rely on the density notch to determine which density filter to use.
I have only one 1.44mb drive - a Sony MPF520-D P/N 04H7405 FRU 93F2361 - that does indeed respect the density line, thereby allowing a SCP or floppy controller to override a density notch. (Note: some drives use the density line to engage the 360RPM Japanese "Mode 3") I think that drive was used in IBM Valuepoint systems in order to maintain compatibility with these disks. It is otherwise a perfectly standard 1.44mb drive.
It is likely that some 720k disks may not have been capable of holding a high density signal, and have degraded over time. These will not be readable in any drive, no matter what. The magnetic formula used to make the disk cookies differed between manufacturers. That said, I have many 720k disks that were intentionally notched, successfully used as high density, and are still readable today.
I don't have any sample disks like this off hand, so I can't check - but It is also conceivable that some PS/2 systems may have written something differently on the boot sector or file system that may have prevented *DOS* from reading them properly. But in that case, a flux level reader, once the drive's density filter is set properly, should still be able to read the content. Make a dump, feed it to HxC and the sectors should light up green.