installing 10.13 on external HDs
i have 3 external mechanical HDs [clones of my 'boot' 10.12.6 HD] attached via FW800 and all 3 FAILED to upgrade to 10.13
I even ERASED 1 drive to see if that helped - didn't
my mac laptop is listed as being OK to upgrade to 10.13
this is the way i 'test' new macOS before i commit to the new OS
MacBook Pro, macOS Sierra (10.12.6), MBP 17 2.53 i5 8, GB RAM 480 GB SSD