the windows partitioner dosent want to let you do that because Page fileing may be active on the drive, i had the same problem, and turned off page fileing, i still had the problem.. google search "****** partition manager", this is a 3rd party partitioner that is better than the default one (im my opinion).
Edit:
the forum blanked out the company name... but if you still need it, you can send me a personal message (this will work for sure, trust me)