Is my plan a good idea?

[rsberzerker]

Rainbow tables are virtually useless against salted hashes. And even if they weren't, you have to factor in disk access times: How fast can you search those tables vs. how fast is hashcat (and oclhashcat). This is made worse by the fact you specify an external HD. Does your computer have USB 3.0 ports? Is the HD a USB 3.0? Even then, based on my experience, don't expect more than about 30MB/sec transfer rate and that's on a straight copy.

You would be better off spending the HD money on a better video card.