I found with my mac and card reader that you couldn't delete from the card (the file is deleted but the memory isn't freed) I know that it is possible to use the terminal to get around this, but it's a chore. Therefore you really need IGP32 to format your cards every time you re-use them.
My mac wouldn't recognise the GP32 with IGP32 to begin with, so I had to use a PC to format my card, install IDrive, and then with IDrive on the smc my mac could recognise my GP32.
After the device was recognised, my mac could communicate with my GP32 via IGP32, with or without IDrive being on the smc...

Hope this helps.
BTW: I use Mac OSX Panther on a G4 IBook.