So the drive cannot find the MBR or it is corrupted.
On each drive there is a master boot record and a shadow copy.
This is in case the original is corrupted making the drive un boot able.
The two messages should be no boot able device found. Or no boot able os found.
What OS are you using ?
Simply schedule a scan disk in windows with the drive connected. if windows 7 ect.
With windows xp the boot of the drive can be fixed by command prompt. via a windows xp installation disk.
Once at the dos prompt type: Fixmbr
And : Fixboot.
That should restore the drive to boot able if an Os resides on it.