External Device Trouble

i recently bought a macbook, but im having a bit of trouble with using external devices after ive connected them to the mac

i have a Seagate FreeAgent desktop drive and a Creative Zen Vision:M that i use with my pc, and mostly out of curiosity, i connected them to my mac.
but now they arnt recognised by my pc

i know that macs use different formatting types with hard drives, but i was wondering wether mine could have altered either of the devices in some way, so that they now wouldnt work with a pc?

thanks a lot