It's an obvious tip but make sure that you format any new HD's you buy and use with your FCP system.
Make sure they're formatted for Mac if you're using them with FCP. Journaled is optional and up to debate (I don't journal my media storage drives but they are RAIDed for back up) but make sure they're not formatted as FAT16 or you'll end up with captured files that end in -av1 -av2 -av3 etc...which can be a huge hassle later on if they're not handled carefully. They also can't store files larger than 2GBs.
Oh, and don't buy External USB drives, either.