Another reason for your problems can be that the floppy disks were originally formatted with FastFileSystem. If your A500 is a standard version, it will have kickstart 1.3 installed, and if this the case, the problem will be that it can't by default read FFS formatted disks.
The solution is to add a new entry in the Devs/Mountlist file found on your Workbench disk, where it uses FastFileSystem instead of OldFileSystem.
I found this old thread with more info regarding this:
http://www.amiga.org/forums/showthread.php?t=7630
