i've never encountered a problem with Windows 7 wants to format a hard drive... what happens, exactly? is there some kind of balloon on the taskbar or a window that pops up? this notification could probably be disabled somewhere but i have no idea where, possibly disk management?
perhaps there is some left over Vista partition information on said drive which 7 wants to convert, but that's going out on a limb. i guess if you performed an upgrade instead of a full install there might be some bugs, but i can't seem to find any information on this issue. if the drive in question is a secondary storage device, it could be configured incorrectly. can you access the files on the drive?