The problem is that there hasn't been a new H300 bootloader released since I made all targets support large hard drives. The actual Rockbox build for H300 will support it just fine, but the bootloader doesn't, which is a problem

There's an unknown problem with the current H300 bootloader code: if you build and flash it you will get a bricked player because the current code just doesn't work. Until this is fixed by someone with the facilities to recover bricked H300s there's a problem

The thread linked above has references to some old H300 bootloaders which are old enough that they aren't broken but have been patched to support large drives, but you flash one of those at your own risk...
As far as I know there is no such thing as a hard drive which will only work in certain players. People keep claiming otherwise and so far they've all disappeared without giving any useful information or it's turned out to be down to large physical blocks without working emulation, which just needs a different Rockbox change to work.