My Book loses connection to My Cloud

I’m not sure where I should post this so if this is the wrong place please tell me where it should be or move it.

I have a 2TB WD My Cloud running as a NAS with a 3TB WD By Book plugged into it as a backup drive. I gave up on getting the WD software to back up the My Cloud on the My Book years ago and installed Karen’s Replicator on my main computer (win10) to do a file by file backup every night.

This worked perfectly for several years but I recently noticed that KR often reports a lot of failures and eventually figured out that when it reports those failures the My Book seems to have disconnected from the My Cloud.
Sort of.
If I browse to Network>WDMYCLOUD the My Book is in the list of folders (shares) as “My_Book_1230-1” and if I click it I see the folders listed as they should be but when I click one of them I only see 2 or 3 of the folders that should be in it.

I have found by trial & error that if I unplug the power supply from the My Book, wait until the drive stops and plug it back in (the only way I can find to re-boot it) the light on the My Cloud flashes for a while and then I can access the files on the My Book and KR can run the backup without errors.
But a day or 2 later KR lists multiple failures again and I have to repeat the process.

During all of this the light on the My Book glows dimly and sometimes flickers a bit (I don’t remember if it was brighter when I first got it - to be honest, once I had everything working I didn’t pay it much attention).