In some case, the amiga will try to use the last cartridge rdb with the next one (diskchange not working well on some configs :-( although it has always worked well here :-? ), so if you insert a Mac or PC formated one in the syquest drive it will say that the rdb is corrupt or invalid and you get a NDOS icon :-) if it is the case for you don't forget to make a "diskchange syq:" (or whatever you've named you syquests drives) each time you change a cartridge for another or you can corrupt your datas...
Now to mount the Mac or PC cartridges you need CrossDos or CrossMac ConfigDisk utility or make a mountlist (you need to know how it has been configured though).
another good package is fat95 for PC formated disks (available on aminet) :-)