Anyway to use crypsetup, you first must have a free partiton on a disk. In this instance I am using /dev/sdc1, which is a freeagent external usb drive.
First initialize the LUKS partition. My target is /dev/sdc1
#cryptsetup luksFormat /dev/sdc1
Then open the LUKS partition setup the dev mapper device. The command below creates /dev/mapper/freeagent
#cryptsetup luksOpen /dev/sdc1 freeagent
Create a passkey file if you want the device to be able to automount at boot.
#touch /root/freeagent_passkey && chmod 600 /root/freeagent_passkey
Make cryptsetup aware of the key
#cryptsetup luksAddKey /dev/sdc1 /root/freeagent_passkey
#echo "mypasskey" > /root/freeagent_passkey
Dont forget to make a filesystem
#mkfs -t ext4 /dev/mapper/freeagent
Then add the following to /etc/fstab…
/dev/mapper/freeagent /freeagent ext4 _netdev 1 1
And add the following to /etc/crypttab. Note that the first entry is the name of the /dev/mapper device
freeagent /dev/sdc1 /freeagent
To get a status on a device and to see the mappings between /dev/mapper and /dev/sdc1
#cryptsetup status
/dev/mapper/freeagent
/dev/mapper//dev/mapper/freeagent is active:
cipher: aes-cbc-essiv:sha256
keysize: 128 bits
device: /dev/sdc1
offset: 1032 sectors
size: 2930270970 sectors
mode: read/write
Make sure you keep track of when to use /dev/mapper/freeagent vs /dev/sdc1 in the commands above.