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The simple rule of thumb is to look for a USB 2.0 drive ad it should work with any modern (ie. upto 5 years old) computer system running Windows. Most drives will, in some way or another, run under OSX and most flavours of Linux.
Though it makes a difference with external hard drive transfer speeds, IEEE1394 aka Firewire, has little benefit on an external CDRW or DVDRW drive, so it's not worth paying extra for.