So are your 2 drives still in a RAID array? If so, BT5 should install fine in a dual-boot situation. Personally, my BackTrack installs have gone 1 of 2 ways: either format the whole disk for BT, or dual-boot. In both instances, it's been either 1 physical drive, or a 2-drive RAID array. If you're using 2 physical drives, not in a RAID array, make sure you've got one set as master, and the other as slave.