I'm not sure if this has been mentioned already (I don't remember reading it).
If they are HDs then you'll need to put a bit of sellotape over the small hole on the opposite side of the label of the write protection hole.
The PCs should then be able to read it as a DD disk instead of a HD. The Amiga can usually read them because they don't have the HD sensor pin thingy. PCs do.
I know it's a simple thing, but they can easily be forgotten :-)