I had an issue of having Content Scan not completing a full scan. I read many comments on this. Some said to use SSH to disable something. Some said that would take days to finish. So on etc…
The simple thing I did was to do System Factory restore for system only. And I performed X-Search (freeware software out on the Internet) to search for all of .db files and deleted them. After this, it will do MEDIA SCAN for like one hour, then will fast index thumbnails. I was able to complete the whole thing in four hours with 55,000 photos.
Try that if you cannot get Content Scan to be completed.
Get X-Search and search for all .db files.
Delete all of .db files (mine was 757 files)
Perform System Factory restore for System only (will keep your data)
Set up your My Cloud again.
It will do MEDIA SCAN for like one hour.
It will finally do Content Scan which will take a few hours to do.
Do not use My Cloud device for anything until the scan is completed or Content Scan will give you an error. Happened to me once.