External hard drives have proved to be less reliable than internal ones owing mainly to the fact that they have a adapter-board attached to them (SATA-to-USB) and it's often this which fails, not the drive itself.
When that happens, the only fix is to extract the drive from it's enclosure (not an easy job), remove the adapter-board, then put the drive into a third-party enclosure.
However, that's sometimes not possible because the manufacturer of the drive may have given it a non-standard, custom interface which is neither IDE nor SATA.