I fixed the problem.
First I copied the Win7 bootloader files from H: to C:
Then I used
to create a MBR on C:
I then disabled the H: drive in the BIOS and made sure the C: drive was the at the top of the boot priority list.
I then booted from my Win7 DVD and selected Repair which quickly detected errors in the boot loader and fixed them
That enabled me to boot Win7 OK.
I then reenabled the H: drive and in the BIOS and rebooted.
The H: was no longer marked as the System drive, allowing me to delete the old boot loader files.
I can now backup to H: