I thought that "switching" the P2 store from PC to Mac only changed the interface, not format the HD. It's FAT 32 regardless so there wouldn't be a need for it to wipe out your footage in making the switch. However... I do not have a P2 Store so don't try it on my word!!!!! Other thoughts,
Using Boot Camp to get the footage off.
Finding a buddy with a PC to offload.
If you have a PC that you can offload the P2 Store with, it's probably safer. The Mac has a tendency to write to all media and create hidden system files on it, and there is also a unresolved problem with ingesting material from P2 media directly into FCP.
I'm not saying that you will always run into problems when using a Mac to offload, but it seems to happen more than it should.
And yes, you can drag the files to another drive for importing it later.