After backup of a SYSTEM drive attached via USB with Acronis 2013, get BOOTMGR MISSING error.

Hi,
Today I tried to make a backup of a working system that has a RAID1 configuration (mirroring).

I took the working drive out of the working system, attached it via USB to the computer that had Acronis True Image 2013 on, and did a full DISK back up of the entire hard drive.

When we reinstalled the freshly backed up drive into the RAID1 system, we get "BOOTMGR MISSING, press CTRL-ALT-DEL".

Any help would be appreciated, and thanks in advance,
David.