restored drive not bootable

I'm running centos 7 64 bit server with mondo-3.2.2-1.rhel7.x86_64.
I have a 120gb ssd connected by usb and it appears as /dev/sdb.
I can create a backup of a centos 7 server drive to the usb drive
using this command:

mondoarchive -OU -d /dev/sdb -s 100g -G

I can also boot from this same usb drive and run a restore using the
'nuke' command but the resultant restored drive will not boot. I
either get a black screen with flashing cursor or a GRUB error. I've
tried doing a repair install on the restored centos 7 drive but the
repair process can't find any linux partitions. There's no boot record
on the restored drive and nothing to go on. Does the MBR have to be
restored seperately? How do I get the drive booting again? Thanks for
any help.

------------------------------------------------------------------------------
Check out the vibrant tech community on one of the world's most
engaging tech sites, Slashdot.org! http://sdm.link/slashdot