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