Macos High Sierra, Digikam 5.7, Macbook Pro.

I feel sure that I must be missing something and so request assistance please.

I have photo's on a USB drive and have created the database and I can see the thumbnails database which is not on the same drive as the photo's.
I am browsing the thumbnails and deliberately unmount the usb drive thus removing access to the photo's.
The thumbnails are still browse-able.
I shut down Digikam and re-open only to find that the thumbnails are no longer available.
[EDIT] In fact on restarting without the disk the thumbnails do appear briefly and the activity shows 0%, a few seconds later the activity jumps to 100% and the thumbnails disappear as do the numbers against the keywords.

I deduce that if the database is available and useable after I removed the drive then Digikam can browse without access to the photo's and it appears that the only difference between the two scenarios is that in one the thumbnail database is already loaded and in the other it only requires loading.

It appears to me that since the difference seems to have no regard to the actual photo's or the disk they are on, the problem must be with me.
I can see no reason why Digikam would not be able to simply load the database.
[EDIT] The Tags do show and indicate how many photo's are included.
Thank you.