It’s coercivity of the magnetic medium. HD disks have higher coercivity than DD which means higher write current is needed to put phase transitions down on the

It’s coercivity of the magnetic medium. HD disks have higher coercivity than DD which means higher write current is needed to put phase transitions down on the magnetic medium when recording. There are also some differences with track widths which complicates things further if you try to reformat from HD to DD without first degaussing. For 3.5” disks the difference is small enough that you only tend to run into problems if you format a disk as HD in an HD drive then try to reformat and reuse it in a DD drive as DD without degaussing first (because the DD drive won’t be able to fully remove traces of the previous HD signal). For 5.25” though the difference in coercivity is quite a lot bigger and even using freshly degaussed HD discs in a DD/SD drive will very likely lead to problems quite quickly.