Then look at the cables because some cables are made cheaply these days, even from so called manufacturers of externals! If they were made before 2015 were made to fail after a few years! Also if this traditionally external make sure the drive it are still good bad use most plater drives fail...
Depends did you probably unmount these drives at one time? Also look at if you have a bad cable or the drive is not blessed to boot from that drive or it has password on it!
Sometimes in especially in upgraded version checking in System Preferences, Privacy and uncheck, reboot and go back in and out a check mark back in helps because there seems to be something (I think it was the built in firewall that stops it! To me it looks like using that firewall seems not...
To rebuild your Spotlight search data especially after add a networked NAS old DAS NAS! Just follow the instructions is Apple Support data web page: Rebuild Spotlight or use the Terminal command: sudo mdutil -E / and it takes about 20 minutes to complete!
Also I have a tip for you! There is domination ware called Onyx and if once every three months run all the ‘Maintenance’ routines and let the program reboot your machine to keep in running well! Also if your not a Thermal dude then consider the free program ThinkerTool!
I have read in an Apple Discussions thread thread talk about Time Machine not backing up was do to a Mac OS Firewall issue is involved so turn it off allowed the backup to act normal again!