GNU/Linux: "device mapper: create ioctl failed" "device or resource busy"

I can't mount my encrypted devices anymore. The error is: device mapper: create ioctl failed device or resource busy This error arises both with two different programs to access TrueCrypt encrypted devices: TrueCrypt and Tc-play. In this case, the recommendation is to remove /dev/mapper/truecrypt* directories, or look for processes that are blocking the device. However, there is no /dev/mapper/truecrypt* directory, and lsof returns nothing. One TrueCrypt device takes a whole HDD. According to fdisk, this partition is formatted with HPFS/NTFS. Another TrueCrypt device is on a partition on /dev/sda. According to fdisk, this partition is "Linux" (ext3 or ext4, if I remember correctly). What could be causing the error? Software: - Debian GNU/Linux 6