In Win7, I have 2 internal hard drives. C:\ and M:\
After a re-install of 7, the system would not recognize M:.
Only C:\ and D:\(system reserve) were showing in Disk Management. Somehow, I wound up making M: the 'system" drive and the PC now boots from M:
How can I change the system to boot from D:\ partition on the C: drive
Switch back to the way it was, all you were missing is the assignment of M: to that particular drive. Now that you have it set on M:, make C: your default system drive or D: if that is where your system is (backup files are not boot files).