[mitch13]

Due to the number of books/magazines/notes I have in Calibre I decided to split it into several libraries. New items generally are added to my main Calibre library then distributed to other libraries as required. This has been working very well.
In the last couple of days something has changed (probably by me in error though not that I remember).

I am running the latest version of Calibre (3.42). The only thing that happened recently is that Windows 10 decided that the USB hard drive that Calibre is on needed 'fixing'. I am beginning to fear for this drive but it comes back OK.
Now, Calibre loads OK and finds my main Calibre library. However, although all the other libraries which are in the same Windows folder, they are no options to either send books to them or as a quick change/search under Calibre Library icon.
I have been using Calibre for years and multiple libraries for several years but this problem is a new one to me. I have searched for multiple library queries but found nothing relative to this.

Any suggestions welcome. I have run maintenance and the initial run comes up clean - no errors in my database. The second run which checks files is another story - heaps of errors but I don't think any of them relate to my main problem.