Before using your disk partition you will need to format it with a filesystem. The common file systems under ClearOS are:
Different partition types have their own strengths and weaknesses. With a partition in place you can place a filesystem on the drive.
EXT is a common filesystem that has been around. When EXT version 3 came out, journaling was added and that increased reliability for operating system use. EXT2 is still used for some flash devices since journal activities require more writes.
Mounting ext3/4 filesystems will commonly reserve a portion of the disk, if you wish to reclaim this space, run:
tune2fs -m 0 /dev/mapper/data-data0