RAID-1 array: one drive is now "missing" out of nowhere

[Turbonium]

I booted up my desktop for the first time in almost 3 months just now (it was off and unplugged the whole time), and it turns out one of the drives in the RAID-1 array is "missing" (likely dead or dying), which is very odd, because my system was working just fine the last time I used it.

Has anyone ever heard of a hard drive essentially dying while powered off like that? Or do you think it just died during bootup? Perhaps it was somehow linked to being off for so long. Or perhaps the motherboard is dying?

I find this rather odd. Note that I did not in any way move the desktop. It's been sitting there the whole time. Nothing (like say a SATA cable) could have been dislodged internally.

 

[Fardringle]

It's entirely possible that the drive was dying when you last used the machine. It was still working enough to function while it was still active, but when you shut the computer down, that drive was no longer able to power up properly again.