I'm pretty sure you can't install the Lightroom software on an external HD. Lightroom has to be installed on your system (boot) disk.
DJ is correct in that the software needs to be run from the boot drive.
However, I believe you could boot your computer using Sierra on the external HD.
You would need to first uninstall LR from one of your other start up drives on the MacBooks because you can only have one subscription for 2 computers.
Then restart using the external with Sierra on it.
Then you would need to log into your Adobe CC and download LR on that boot drive.
Finally run Lightroom on the Sierra Ext boot drive.
Let us know if that works!
I am pretty sure you can run the executable from a different drive than your boot drive without any issue at least on Mac systems. However, Lightroom will load the preferences from your boot drive and won't know it is licensed to your machine and will want you to relicense which won't work because there are already two licenses checked out (this is the permanent license right?). This is no problem with the CC version where all you need to do is to login to CC but will be a problem with the non CC version of Lightroom. You might be able to just copy the license files and preference files over from the internal disk and it might work just fine as it still is the same hardware.
Sierra is installed in the external HD. Even thought it's external, it still is a complete operating system.
LR 4 has been on it. All my other important programs are on it.
I'm simply trying Sierra before putting in on internal HDDs. This is always the way a test a new OS.
Adobe is requesting I un license one copy first. I really don't need to. Never have had to before....
Just need Adobe's input.
>Just need Adobe's input.
You're unlikely to get it here. This is a user-to-user forum and there are only a few Adobe employees on here. Anyway as far as I understand you can only activate the permanent license on two machines at a time. The new install on that Sierra disk is seen as another machine by the creative cloud app so you can't get it to work without deactivating in the other OS I think. You might be able to get some help from a chat with Adobe but they will likely just reset your activations so you have to reactivate on all your installations and it won't solve your problem therefore.
We test an external drive with a new OS all the time before installing a new OS on our main boot drive.
Just startup the computer from the external drive, install LR on it and test it.
We have the Creative Cloud version with makes things easier to download.
If you cannot install LR and the message says you have already activated twice, you may need to "deactivate" Lightroom on one of your other Startup disks.
SO restart with your main boot drive, and "deactivate" the application.
DO not "Uninstall" it.
Then reboot from your Sierra External, install LR and test it.
Here is a link to information on activating from Adobe:
Hope this answers you questions !