Over many Years of fixing people's broken PC's the most common problem with External USB drives was due to due to constant plugging and unplugging breaking the solder joint on the connector on the internal circuit board, not usually a problem with the more upmarket brands. Never yet seen an actual drive that failed nearly always the socket and very rarely a failed chip on the circuit board.
Remove the drive from the enclosure and plug into a PC's SATA or IDE port to test and the drives are fine, easiest fix is a new enclosure or dig out the soldering iron and reconnect the socket to the PCB!