Bootmgr changed drives on its own?

[KnightStalker]

Right heres an odd one, Ive had windows 7 installed for about 3 months on my OS drive, and just today i got the Bootmgr missing thing - now nothing changed apart from a new CPU, so went into the bios and selected my 2nd hdd in the priority list and wham it boots...but how and why has the bootmgr moved drives seemingly on its own and how on earth can i move it back, as there is nothing obvious on the 2nd drive to indicate a boot sector etc..

Edit yet in disk managment, it shows the first HDD as having the boot and the 2nd drive doesnt..yet the 1st drive wont boot but the 2nd will...odd.