I think the question is pretty much self-explanatory, and requires a yes or no answer.
My specific situation is the following. I recently replaced my old HDD with an SSD, upgrading to xenial in the process. I placed the HDD with trusty in an external USB case, and tried booting from it but it didn't work.
I am wondering whether I need to add some files to the HDD in order to make it externally bootable, or whether this is a hardware problem as suggested here (which btw I think is not a duplicate). I am using an ORICO 2588US3 external case on a Toshiba Satellite L855-149.