It is hard to say what is causing the problem with multiple drives. Unless you are performing a low level bit for bit (or block for block) clone, then file system issues are not going to be transferred (I'm not aware of any macOS app to do this). I'm not certain what happens if "asr" is used to "clone" an APFS volume or snapshot (utilized by Disk Utility "Restore" and third party cloning apps...at least for boot volumes).
I personally have no faith in the APFS file system at the moment since I have seen so many instances both with our organization's Macs and posts on these forums where the APFS has errors that cannot be repaired by First Aid. No idea what is causing this since theoretically the APFS file system should be less prone to issues due to how it works. Unless it is a Time Machine backup drive, I make sure to use HFS+ on my external Mac drives since it is more stable in my opinion (it took many years for HFS and HFS+ to stabilize as well).
Like the others have mentioned, I don't have much faith in WD products (over the years either they work really well or they are nothing but problems and you never know which drive you will actually get), plus WD has been known to play games & trick people so I try to avoid them as much as possible.
Make sure to connect the drive directly to the computer since any docks, hubs, adapters may be interfering somehow and creating issues with the external drives. Another consideration is the drives (or any item in the chain) may have a hardware issue of some sort (perhaps even the Mac although I would tend to expect a similar issue with the boot drive).
As for the drive not unmounting, that can be tricky to figure out. Many times Spotlight may be indexing it. Other times an app (even the Finder) may not detach itself even when you close out the window and the app. The only way I have discovered to even attempt to discover the app or process still attached to a drive is by using a command line utility such as "fuser" and/or "lsof", but even these don't always help. Some times I just resort to powering off the computer so I can unplug the drive.
