fwick 0 Newbie Poster

I have been using an ext USB drive with no problems on my powerbook with 10.3.9.
I needed to transfer a few big images to a win machine, after plugging it in there (unsuccessfully) I returned the drive to the mac and now it says "you have inserted a disk containing no volumes that OSX can read...." How can I get at the files? AFAIK nothing was written to the disk when it was plugged into the PC, could it have been changed???
HELP HELP HELP!!!:'(