Can I use a regular external hard drive on an Xbox 360?

If it specifically says you need an Xbox hard drive, no you can't use an external PC HDD. The 360 lets you format USB devices which are usually flash or externals to use up to 16GBs on each for storing game data. Normally you could store expansions on an external, but if it's saying you need a 360 one, it means an official hard drive. USB drives wouldn't work for that.