Win 7 can boot from any drive letter, it's actually meaningless.
Windows only sees it as a device number and partition number i.e.
drive 0 partition 1
It doesn't even see the drive letter in the bootmgr.
It sounds to me like the real problem here is grub. I know from personal experience it can be nearly impossible to get rid of.
You could just try a startup repair. Startup Repair