bootcamp won’t load windows from external superdrive

my internal optical is toast so I got an external that has been working fine.

bootcamp recognized my windows DVD when setting up the partition.

but when restarting the drive spins for 5 seconds then stops.

I get “no bootable device - - insert boot disk and press any key“.

has worked in the past with older OS and the internal.

what is causing this and is there a workaround?

thx, ric.