I have a Windows Vista system which is going on three years old. I am running an ASUS work station motherboard and the machine has been very stable and reliable until Saturday.
On Saturday morning according to event viewer at 6.09 am my secondary SATA HDD dissappeared from my system.
After rebooting the drive reappeared and stayed available for several hours before vanishing again.
After a reboot on Monday it reappeared and stayed available until Midnight. It was accessed by my Media system and successfully recorded some TV shows until just after Midnight it suddenly disappeared again. I have not restarted yet, but anticipate the drive will reappear again.
I know this can be caused by a failing HDD however my gut feel is there is something else happening.
Well some stuff he said was right, you can check the connection and make sure it's hooked up properly. If it still persists, you can scan your HDD with Chkdsk to see if there are any errors (which may help determine if the HDD is failing).
Guessing all the connections are in place correctly? Simple take it out and replace it. Also clean out the PC if you can, maybe dust is affecting the inside...or maybe overheating is causing some conflicts. Also have you tried the HDD on another PC and see if it's disappearing also?