External drives are (with very few exceptions) internal drives fitted in an enclosure, with a bridge chip to convert the native IDE or SATA interface of the drive to USB. These bridge chips can interfere with certain operations, such as bitsetting. Others are fine, it all depends on the combination of bridge chip & drive.
If this is the case, the only solution is what Liggy suggested - removing the drive from its enclosure and connecting it directly to a computer via IDE or SATA (or chancing your luck with a different USB bridge). Unfortunately with laptops this may be very difficult or impossible.
Another possibility is that software on the computer is interfering with the process. Anything which intercepts commands going to drives, such as DVD/Blu-Ray decryption software running in the background (such as AnyDVD, DVDFab Passkey), can cause problems.