Thank you both for your suggestions.
I tried a variety of things but nothing worked. Then for some reason I right clicked on my external hard disk and released on Get Info (Command I, this is a Mac shortcut).
I noticed that name of my external hard disk (as it appears on my Desktop) and the name of it in the Gen Info window were different! (The number "1" was appended to the name in the Get Info window.)
(I think this happened because I unplugged the hard disk once without ejecting it first...)
I tried deleting the 1 in the Get Info window but the change wouldn't stick so I made a new folder in the external hard disk and moved my Calibre library into it.
I then launched Calibre and as it could not find the library, it created a new database.
I then quit Calibre, deleted that "empty" database, then moved my proper library back to its original folder (where the "empty" database had been located).
I then launched Calibre and all my books were once again visible...
I hope this will be useful to someone with a similar problem.
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