>>You were certainly right on that suggestion. The event logs showed many errors, including quite a few disk i/o errors. I will never know exactly how or when the problem began but I still suspect it was associated with the backup software. I ended up purchasing a new HD and restoring from an older backup, then copying only the newer files that were truly essential. The procedure was probably overkill, and took a week to fix (formated the new disk at least 3 times to get the restore done correctly), but everything seems to be working now. The down side is the lost productivity and stress; the up side is I have lots of disk space now :-)
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>Glad you got it sorted.
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>These days (mechanical) hard drives are incredibly cheap. If you rely heavily on a particular machine it's a good idea to have at least 2 hard drives in a mirrored (RAID1) array. Everything you do is simultaneously copied to 2 separate drives, not just one. If one fails, you're still OK until you can get a replacement drive and return to full redundancy. These days most new desktop motherboards support hardware RAID directly, as do the majority of boards less than, say, 3 or 4 years old.
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>It's worked well for me; I've had 3 complete drive failures of WD Caviar Black 1TB drives since Feb. 2010, but zero data loss from any of those failures.

Ah, that proves to me that those drives are not reliable. If it occurs that frequent, I'd not feel comfortable despite the RAID config.