Could you expand on the History of this drives use - was it the previous OS system drive? to which you have added win7 on the current drive?
If so the drive could still contain the boot files for your current install - if this is so then Windows is preventing you from deleting these files, which are essential to run your current install
Please provide a readable screenshot of your disk management to allow assessment of the full situation.
This sort of thing is not uncommon and is repairable, but does need full information to ensure you retain use of your current install