SOLUTION - I have experienced a USB external hard drive that is not recognized in "Computer" and is therefore not accessible. The drive does appear normally in Device Manager. Sometimes an external drive which has been in normal use shifts into this situation. In my case the drive appeared to have lost its "designation".
Other brands of USB external hard drive plugged into the computer are also not recognized, but flash drives are recognized.
The external hard drive appears normally and works when plugged into other computers.
To recapture the drive designation on the computer experiencing the problem -
Go to Contol Panel - Administrative Tools - Computer Management - Storage - Disk Management. In my case the drive appeared, but has no designation letter. I right clicked on the "blank" drive and then
selected "Make Drive Active" and "Change Drive Letter and Paths". That seems to resolve the problem.