Re: Western Digital
I am going to give a short lecture on UPS and equipment hooked up to them...
Most generic UPS use Stepped Sine-wave, and most small electronics only needing a little bit of power will use a transformer, and some smoothing capacitors to provide power for the device, the drawback is that stepped sine-wave can cause serious damage to the power adapter, or equipment hooked up to the adapter.
Even Stepped Approximation Sine-wave (VERY close to true sine-wave) can cause issues, but it's not as likely, and most people wont splurge the extra cash for an UPS that gives true sine-wave output, seeing as the cheapest of the true sine-wave units is around $250USD
The next issue, if you had a cheap UPS, it probably doesn't have a fast switch-over time, basically, how fast it can switch from house mains to backup, sure it might be ok for low-power draw devices, but anything demanding power will just turn off.
Hate to say it, but I want to say that your HDD died due to voltage spikes from where power went out, UPS kicked on, and at the same time the PC turned off... Could have seen spikes from either the AC side, or even the USB side of the external disk.