Not only am I experiencing the issues discussed in this thread, but I'm also still experiencing the Photo>Edit In>...Adobe Photoshop CC 2015.5... fun wherein the copy of Photoshop that's on my backup drive is the one that opens.
this issue is not new, has existed for years, and Adobe doesn't seem interested in fixing it. They always blame apple (or Carbon Copy Cloner) for the problem, which seems illogical because the correct copy of Photoshop to open is nearly always in the same application folder as the currently running copy of Lr. they could even change the preference for external editing so you could specify a path to the "default" copy of Ps.
I had a long email conversation with several at Adobe, and supposedly the issue was escalated to engineering, but this was probably two years ago, and no change.
As you have discovered the only sure solution is to unmount any backups that contain the photoshop app. Another solution which I use if your backup program allows it is to exclude the Photoshop app from backing up. After all if you need to reinstall it you can do so easily from adobe, so all I you really need to backup are the support files in the Ps folder.
