Scanning with consumer drives is not accurate. So I recommend same drives which many CD/DVD geeks use. In short, LITEON and BenQ (DW1620, 1640, 1650, 1655 only).
IDE <-> USB2.0 converter is available with all internal LITEON drives. This 'pseudo' external USB LITEON drive works fine for scanning. I hear internal BenQ drive is slightly difficult to convert external USB drive, so you should buy genuine external BenQ drive.