>>>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.
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>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.

Anyone doing serious work on a computer should also have regular file-based and image backups. Those should also include offsite or cloud-based backups, to cover the case of total loss of computer and/or backup media due to theft, fire etc. There are only two types of computer users - those who are true believers in backup, and those who are about to become true believers.

I have multi-level disaster recovery in place. AFAIC RAID1 is just one of those levels; having gone through several drive failures I know it works. I actually feel more comfortable than if I'd implemented RAID1 and never had a failure, in which case I would not be sure it works. YMMV.