Only one specification: you need to be able to connect it to both, preferable via USB3. USB2 is slower, most PC's don't have Firewire or eSata.
And only one instruction: format it FAT32.
I want to secure my photos by backing them up on a hard drive. Is it possible to save them from my mac, yet read them from my pc? If so is there a specification that I need to be sure my hard drive has? (I haven't purchased one yet)