Comment 21 for bug 725408

Yep, that's what is going on. The ntfs partition is listed as /dev/dm-2 by mount, and remains mounted. Attempting to umount it by the /dev/mapper name, OR /dev/dm-2 fails saying it is not mounted. To unmount it, you have to umount the directory where it was mounted. If I remove that ntfs partition, then the installer proceeds normally, so somehow that stuck mount is making the whole partman process hang up.