A couple of days ago I decided that having only booted into Vista once since recieving my copy of 7, it was time to format the drive.
Cue lots of cursing and swearing - it seems I can't as the Vista partition has the bootmgr. I get EasyBCD and copy the loader on to my 7 partition, reboot and try again, but I still have the same problem.
In disk management the two drive have the following attributes :
Vista Partition : System, Active, Primary Partition
7 Partition : Boot, Page File, Primary Partition
Trying to format results in "Windows cannot format the system partition on this disk."
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Just as I was going to post this I noticed the 'Mark Partition as active' option for the Win7 partition. Problem solved.