Installing a portable hard drive to more than one computer?

I recently got my first portable hard drive, a WD Elements. My friends said that they didnt even have to install there hard drives, they just plugged it in and it would work like a normal USB so that is what I expected. Except for mine, I realised I had to install it. So I partitioned it and everything for my Macbook by using support.wdc.com, but what happens if i wnat to drag files onto my hard drive from another computer? I have a PC desk top and I want to drag some movies from it onto my hard drive. I tried installing it onto the PC as well, but it wouldn't work... any help?