It is important to regularly backup system and user data stored for example in
/var and for server installations, also
Using Btrfs snapshots
Using LVM snapshots
Having a bootable backup can be useful in case the filesystem becomes corrupt or if an update breaks the system. The backup can also be used as a test bed for updates, with the testing repo enabled, etc. If you transferred the system to a different partition or drive and you want to boot it, the process is as simple as updating the backup's
/etc/fstab and your bootloader's configuration file.
This section assumes that you backed up the system to another drive or partition, that your current bootloader is working fine, and that you want to boot from the backup as well.
Update the fstab
Without rebooting, edit the backup's fstab by commenting out or removing any existing entries. Add one entry for the partition containing the backup like the example here:
/dev/sdaX / ext4 defaults 0 1
Remember to use the proper device name and filesystem type.
Update the bootloader's configuration file
For Syslinux, all you need to do is duplicate the current entry, except pointing to a different drive or partition.
syslinux.cfg, you can also temporarily edit the menu during boot. When the menu shows up, press the
Tabkey and change the relevant entries. Partitions are counted from one, drives are counted from zero.
For GRUB, it is recommended that you automatically re-generate the main configuration file. If you want to freshly install all grub files to somewhere other than
/boot, such as
/mnt/newroot/boot, use the
Also verify the new menu entry in
/boot/grub/grub.cfg. Make sure the UUID is matching the new partition, otherwise it could still boot the old system. Find the UUID of a partition with lsblk:
$ lsblk -no NAME,UUID /dev/sdb3
where you substitute the desired partition for /dev/sdb3. To list the UUIDs of partitions grub thinks it can boot, use grep:
# grep UUID= /boot/grub/grub.cfg
Reboot the computer and select the right entry in the bootloader. This will load the system for the first time. All peripherals should be detected and the empty folders in
/ will be populated.
Now you can re-edit
/etc/fstab to add the previously removed partitions and mount points.