This determines how the backup is done. The following options are 
available:

auto

Analyzes the partitions on the drive to determine the best of the 
three methods described below. If the drive contains ext4, reiser4, 
or btrfs partitions, fsarchiver will be used. If the drive contains 
ext2, ext3, reiserfs, vfat, hpfs, jfs, xfs, ufs, hfs, or ntfs 
partitions, partimage will be used. For any other partition type, or 
if a drive doesn't contain any partitions, dd will be used. 
This is the default.

partimage

Uses the partimage utility to make images of individual partitions. 
Supports ext2, ext3, reiserfs, FAT16/32, HPFS, JFS, XFS, UFS, HFS, 
and NTFS. Saves and restores the MBR(s) and partition table(s). 
Recreates Linux swap partitions. Works for most configurations.

fsarchiver

Uses the fsarchiver utility to archive the files of a filesystem. 
Supports ext2, ext3, reiserfs, XFS, JFS, ext4, reiser4, btrfs, and 
NTFS. Saves and restores the MBR(s) and partition table(s). 
Recreates Linux swap partitions. More tolerant of drive hardware 
failures during backup. After restoring this type of backup, you may 
have to reinstall your bootloader, since the restored system files 
may not be in the same physical location on the disk. This method is 
the fastest method during both backup and restore due to the 
multithreaded compression/decompression.

dd

Uses the dd utility to create bit-by-bit images of entire hard 
drives or partitions. Works for any disk drive with any filesystem 
or no filesystem at all. The most reliable method. Also the slowest 
method, and uses the most space for backups. Use for drives with LVM 
partitions. If none of the other methods work, use this method.