I'm cutting and pasting from Apple's Community support site where I posted this issue today, rather than type it again, since the details are dense:
My system, prior to 12/20/2017 was a MacBookPro9.2 (mid 2012) running 10.8.5. Why that OS? I hate Apple upgrades, as things always break and 3rd party SW has to be upgraded. Mountain Lion is a perfectly good OS, but over time has not been able to keep up with iOS. I loath the Apple phone so 10.8.5 suited me just fine, until I needed some R bits complied for Sierra. So, R is very important to me so I upgraded to 10.13.2 from the Apple store. I upgraded to an SSD inside the Mac, and after 5 days of testing, used Carbon Copy Cloner 5.0.5 to clone the SSD to an HFS+ (recommended by CCC) HDD inside an OWC/Mac.Sales On-The-Go USB 3.0 enclosure. No issues and CCC even created a 10.13.2 Recovery image. Interestingly, the Apple store upgrade wiped the 10.8.2 Recovery image on the SSD, leaving me with no Recovery image on that drive.
The HDD inside that enclosure is not bootable, neither the standard image nor the Recovery image though it is readable. The boot-time progress bar goes about 60% of the way and then aborts to the circle with the diagonal line. Bummer! After several days of troubleshooting with a CCC support engineer, with zero success, I was left with no option other than to swap the SSD and the HDD to nail down whether it was the enclosure or CCC at fault. Now folks, get this, the HDD when inside the machine boots perfectly, both standard and Recovery images but when the SSD, which received the bits from the Apple store, is inside the OWC enclosure, it will not boot. Also interestingly, the problem is not bidirectional. CCC can write to a disk inside this enclosure and create a perfect OS image, but the error occurs at boot time when the bits are being read. Man, does that make me suspicious. Did Apple do this to harm competitive device makers or is 10.13.2 still flaky? I've reported the issue to OWC but have yet to hear back.
So, other than buying a new external enclosure guaranteed to work with High Sierra, is there any fix to make the OS boot from an external device? Has this issue been seen by others? Also, bummer twice over, I have a RAID backup device still at 10.8.5, bootable, to which I fear attempting a backup, since that image could also be made unbootable 🙁
Best Answer
Assuming I read your question correctly, on the face of it it seems the issue is with the enclosure since neither the HDD nor the SSD will boot when inside it.
However, two things you could try (because they're simple and quick to do) are to reset both the NVRAM and SMC, in that order. Before following the steps below, ensure you have no external hardware connected (so, unplug your external enclosure) and make sure you use the built-in keyboard.
Reset the NVRAM
Older Macs had what's called Parameter RAM (PRAM), newer Macs use Non-Volatile Random-Access Memory (NVRAM). Here’s how to reset the NVRAM on your particular MBP:
Note: When you log back in you may need to readjust some of your system preferences (e.g. speaker volume, screen resolution, startup disk selection, time zone information, etc).
Reset the SMC
To reset the SMC on your particular MBP, follow these steps:
After resetting both the NVRAM and SMC, test your Mac to see whether you can boot from the external enclosure.