|Yes, you CAN diskcopy a 1.2M onto a 1.44M floppy -- BUT you need to be tricky.
The easiest way to do it, is to use DEBUG or a disk editor and copy the boot sector of the 1.2M onto the boot sector of the 1.44M.
Then remove the 1.44M and access the 3.5" drive, abort at the error message. This forces DOS to recognize the floppy ain't there no more. When you re-insert the 1.44M with the 1.2M boot sector, DOS will read the boot sector and "recognize" the disk as a 1.2M formatted disk. NOW you can diskcopy from the 1.2M 5.25" floppy onto the "1.2M" 3.5" floppy.
This trick is useful when migrating old software's install disks to a machine with only a 3.5" drive. I have used it with several versions of DOS on several machines -- I only saw it fail once, on a machine with a strange BIOS that had lots of other compatibility problems.
The "hybrid" disk will still be formatted to 18 sectors/track, but DOS will recognize the existence of only the first 15 sectors on each track. Similar tricks can be used to place a 720k disk image on a 1.2M disk; or a 360k disk image on any of the other standard disk types.
The hybrid disk will work with DOS 3.2 and later, Win95 (original), probably with Win95 OSR2 and Win98, and probably NOT with NT.
The "trick" to all these sub-formatted hybrid disks, is to recognize that DOS loads the info. on the disk layout from the boot sector when it first recognizes there is a new disk in the drive. To change what DOS thinks the disk layout is, you change what is in the boot sector THEN force DOS to re-register the disk.
Several things can get DOS to re-register the disk, but the only way that works reliably under all DOS versions in all cases seems to be taking the disk out of the drive and trying to access it, then aborting the access. Then when the disk is re-inserted, DOS will treat it as a new floppy and register the disk parameters from the boot sector.
Kevin G. Rhoads, Ph.D. (The Cheshire Cat for official Internet mascot.)