Yes, you can. The ones that fit in a standard drive slot in your case are no problem. The external ones may come with a PCI eSATA card that you would want to use. The one I used didn't mention the card in the description, but did come with one and the eSATA cable. The USB or firewire cable they probably come with I assume would work, but wouldn't be fast and I haven't tried that before.
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Xbox 360 & a PC currently undergoing mitosis
•AMD Athlon 64 X2 DC 6000, 3GHz •3GB PC2 5300 DDR2 •Sapphire Radeon HD 6950 2 GB
•Creative SB X-Fi Fatal1ty Pro / Logitech Z-5500 •Logitech G27, Nixim mod