Also, it seems your TM backup is an OWC drive. Is it Seagate "spinning rust" (as joemikeb would say)?
Yes, the TM drive is Seagate and the other, for my SuperDuper backup, is HGST....a subsidiary of Western Digital.
Mine is a straight Seagate drive, plugged into a USB port on the back of the iMac and every now and then I too get the ejection message you mention. Very quickly it re-mounts itself. I have attributed this to a Seagate problem as it does not happen with any other non-Seagate drives I use.
I wouldn't be in a hurry to blame mine on the fact that the drive is Seagate, as it's been plugged into the back of my iMac since 2012 without any issues of any kind. The only difference now is that the Mac recently changed from an early 2008 20" iMac 2.66 GHz Core 2 Duo to a 2017 27" 3.4 GHz Intel Core i5.
