Cheers mate - I'm not brilliant with computers anymore (I'm sure I used to be really good at the techy stuff, now I can barely turn one on!) and tried to format my external one which is about 250gb or so but it refused as it said it was too big.
Just gone and done a fair bit of research into internal ones and I've purchased a new 500gb one, going to pick it up in an hour or so and hopefully will be all good...!
Can't believe I bought a 40gb one. My own fault really.
PSN ID & Xbox Gamertag: Wedjwants