I tried installing Ubuntu 11.10 on my new laptop (HP Pavilion dv6-6b20SE). I didn't want to do a complete re-install and pre-partitioning my hard disk, so I tried shrinking my main NTFS partition (which was about 620 GB + recovery and system partitions) and partitioning the unallocated space, using the Ubuntu setup.
It then showed the 50 GB I tried to shrink was unusable to allocate, so I expanded the NTFS partition back to its original size. Not wanting to give up, I tried it again. Again, it said it was unusable, so I expanded again. After rebooting, and booting up in Windows 7, I noticed about 50 GB missing from my C:/ drive. It cannot be found with disk manager or GParted.