Not hard. Buy a decent external enclosure for about $15-$30. All should have USB, some have FireWire, and a few have eSATA. You can easily buy a cheap hard drive, plug it in, put it together and done!
If you want a portable external HDD, you can even buy a cheap notebook hard drive, a cheap enclosure and etc.
The only reason why I bought my Western Digital 200GB external hard drive was because it was on sale for about $80-$90.
If it wasn't, I would just made my own.