That should work, but he should not have to do that since we are already on 10.11.1 that already has the new drivers. The issue is for some reason his old iMac is not picking up the software update to get to 10.11.1 that would solve this.
Weaselboy, You are right and it SHOULD work, but the way the apple OSX installer works, is that based on the current system you are running it will pull the correct libraries and drivers.
So by running the installer from the new iMac onto the external drive will guarantee the correct libraries.
Another option is to verify that his SSD is formatted using the GUID map and not the MasterBoot Recort.
A rule of thumb, is always created an image/Install on the latest hardware you have. Eventually this image/ Boot drive will be able to boot to any machine.
Regards
All this should not be necessary as I have run the same load for years just updating and moving from one machine to another until this new 27" late 2015 iMac. I got the exact same error screen as the OP, however, I didn't bother with much time troubleshooting it. I just started the setup using the internal HDD and allowed the migration app in ingest the app and user data from the TB SSD to the internal HDD. Once it was complete I made sure I could log in and cloned the internet HDD to the external SSD (while still maintaining the clone copy of my original TB SSD on an external FW800 HDD).
Booted from the cloned created TB SSD, tested successful login and set the external TB SSD as the primary boot disk. Finished, problem solved... until... a fews days later when I let the system update to 10.11.1 (since my original TB SSD load and the new iMac HDD both came with 10.11
My primary user directory is not on the TB drive since it contains much too much data in the way of pics, videos and docs and resides on a separate HDD in a TB enclosure.
Once the install of the 10.11.1 update was complete any attempt to login to my "Standard" User ID failed with a spinning graphic sometimes accompanied by the new beachball (sometimes not) staying on the screen forever.
After a hard restart I logged in from the "Admin" account which resides in it's entirety on the TB SSD and there were no issues. While logged in as the admin user I checked to determine the path to the user directory was still properly set for the standard user. It was.
Annoyed, I restarted, the iMac telling it to boot from the internal HDD (which is no longer in use but has the original point in time configuration after the migration that uses 10.11 and points to the external TB HDD for the standard user.
After the annoying slow startup, (Ugh, I hate HDDs) I was able to login as the standard user. I shut down the system and restarted, selecting the TB SSD as the boot disk and logging in as the standard user. Instead of the forever spinning it continued the post upgrade process asking for the password of Apple ID associated with the standard user.
All of this is to say Apple really needs to do better at testing the various scenarios their users might encounter while attempting to update to the various point releases.
