Unless the system BIOS supports 720k floppy drive, it will assume it's a 3.5-inch one (1.44Mb) so it won't read it properly. Look in BIOS Setup to see which capacity of floppy drive is supported. If 720k is not an option in the BIOS, your 5.25-inch drive can't be used on that computer.
Computer BIOSes stopped supporting 720k floppy drives around 15 years ago, maybe even before that.