Windows had been telling me that I needed to check an external drive for errors, and I did, and it found an error with one file, which it repaired. It indicated no other problems. Then the next time I went to use the drive I got the message above (wording may be slightly different), inviting me to format the drive, which would have lost me some data I hadn't backed up, so I canceled.
I plugged the drive into a different computer and it came up fine in both Windows 7 and Ubuntu Linux. To be safe, I used the other computer to copy its data onto a different drive. Returning then to the first computer, Windows didn't open that drive either, and and I got the same message inviting me to format it too!
I have a dual boot with Ubuntu Linux on this computer as well, started Linux and the drive came up fine. In Linux, I copied the data to a drive which Windows does recognize, and now I have use of my data but not of either external drive in Windows on the problem computer.
Also, the problem computer, a Powerspec G160, has card reader slots and recently, has not been recognizing cards plugged into them. That may be because I changed their drive letter assignments in Windows, moving them to the end of the alphabet so I could keep my external drives at the same drive letters when I plug them into different computers. Maybe this is contributing to the problem with the hard drive?
So, all in all, Windows 7 appears to have become confused about the status of drives and about assigning drive letters on this computer. Can anyone please help me sort this out?
One lesson of all this is that having a dual-boot into Linux can save your data when Windows won't boot or acts up...It isn't the frist time that has happened for me.