I've seen this a few times with external HD connections ..The appearing/vanishing issue was caused in most cases by an intermittent power supply caused by a poor connection in either the USB connector or also a faulty pin connection in the transformer power supply to an external drive. If you haven't done it already..a check on the stability of the connections is the best first step! Let us know how it goes....
the external drive [coolmaster X craft] rebooted and crashed the pc which would get as far as the moterboard logo only, tried del to enter bios but nothing happening nor on reboots or cold power down and restart.
disconnecting external hdd still no joy in boot up. turned poower off at plug and managed to get into bios this time the options in bios to select boot order had disappeared, had to save optimised settings and reboot to get pc working again
about to take it back to PCW
Any idea why it corrupted the BIOS? anr crashed pc