Well here is what I learned about taking your external hard drive and using it back in forth between PC and MAC. BE CAREFUL!!!!!!
I "fubarred" something really well. The drive was not coming up as readable on either machine and both were asking me if I wanted to format it. I panicked. Although I have 3 points of failure on RAW files, most of my edited files are not backed up (tonight this is changing) regularly and I had 3 sessions on it I didn't want to re edit!!! I did my research on the net and learned that probably what happened was the FAT32 got hosed by the MAC and needed repair.
TestDisk [www.cgsecurity.org]. saved me. It's free. It helped repair my damaged partitions and I was able to see and recover my data. If you ever get yourself in this situation check it out. It also works for CF cards too I believe.
They earned a nice donation from me--so worth it.
Pam