External hard drive woes...

The first time I made the switch from Windows to a Mac, I was easily able to access old files and music from my external hard drive on my Macbook Pro. However, when I tried to do the same from my Macbook to my PC at home (via external hard drive), I was unable to read or access the files. Does anyone know how to make sure that I would be able to take files from my Mac, put them on an external, and then retrieve them successfully on a new PC? I'm only asking because I'm selling my iMac in order to build my own PC, and I would be hella pissed if I lost my schoolwork, music, and such. Any help would be appreciated. I'm completely in the dark when it comes to Mac/PC relations. Thanks!