Obviously, once you can access all the files, you copy them to that spare HD you have, then you reformat the old one and - if needed - copy everything back?

I believe you mentioned the possibility of a TB drive. You may have enough "stuff" to make use of one; but having recently (about a year ago) had a backup drive fail while I was putting in the replacement for the drive that failed in the computer, I'd say that especially with that much stuff all in one place you need at least TWO SEPARATE TB drives.

If all of your backups are in one place, you're only marginally safer than if you have no backups at all. The backup drive is no more likely to fail than the main drive, but its also no less likely to fail than the main drive.

My current practice is to use two 250 GB "portable USB" backup drives. The 250 GB is sufficient for all the "documents" on four computers with a total of 6 drives to be all stuffed into either one. Used in alternation, neither backup is more than a couple of weeks old, although I do usually bring them both up to current at least monthly. Using drives designed to be portable (I've got Western Digital Transports, but everybody makes them now), with everything on them, lets me drop one in a briefcase (or in a shirt pocket) when I take the laptop out on the road. I don't usually even bother updating the laptop - just run from the backup while out, and put everything in a \Trip folder for easy synch back home.

My 250GB drives are getting a little toward "full up" now, so I'll probably go at least to 500 GB when I need replacements, but it's really hard for most people to justify a TB size realistically. It likely will wear itself out before you actually need the extra space.

If you do "cumulative" backups, or make a new full-system backup each time, you might actually fill up a TB drive, but if the big drive starts to go flaky you need a separate equally big one to back it up. You're unlikey, I would suspect, to want a TB drive as the system drive in your machine, at least for now(?). And transferring a TB of stuff from one drive to another will take several days even with the best speeds you're likely to get.

Note also that if you use a large backup drive hooked directly to your computer, every AV scan will probably want to scan the big backup. Also, although my Norton Internet Security 2008 didn't, the new NIS 2009 scans all the mapped drives so a full system scan on my computer includes the "documents" folder on Lin's (mounted as my Y:) and the "documents" folder on my kid's computer (mounted as my X:). (But NIS 2009 is much faster, so it actually takes less time to scan them all than it did for just my own 3 drives on my computer with the older version.)

With USB backup drives, I can (and do) unplug them except during backup. Anything that goes on them has been scanned already, on one of the "permanent" drives, so it seems safe enough, and the backups can't be infected if they're disconnected.

John