Compatible External?

Hey guys, I'm a Mac newbie that just bought my first iMac bout last week (still waiting for it to arrive, but that's another rant for another day)

Anyway, I have a 1TB Western Digital External that I have been using for my Windows systems and was wondering if you guys could clear something up for me so that I can put my worries to rest.

This External will be compatible with my iMac with no repercussions, correct? As in, someone I used to know had a MBP and when they connected the external, and then tried to connect it back to a Windows system and it would either wipe or the Windows system wouldn't recognize it. Please tell me this isn't true, as I have a lot of important things on this HD and don't want to lose anything, but still want to move and plug this thing around.

--Shads