Recently the disk holding the root (/) filesystem on one of my linux systems started to report increased SMART raw read error rates, seek error rates and ECC recovered hardware errors.

As these are early indications of a failing disk, it became time to replace the disk.

Normally replacing a disk comes down to plugging in the new one, coyping over the data, umount the old disk, mount the new one in place, unplug the old disk.
But when it is the disk with the root filesystem a couple extra steps are needed.

The steps below worked for my Debian system without problems (even used the opportunity to upgrade to an SSD :-)

(source is this thread on StackExchange)

The following makes some assumptions:

• All commands ran as root when possible
• You are on a physical console to the host (need to type in grub commands to boot up the new disk!)
• You want an ext4 files system
• You are loosely familiar on a basic level with all commands run
• You are NOT booting from a RAID device

So here we go.

1. Physically install new disk into computer and connect to available port leaving old disk in existing position.
2. Boot computer into old OS.
3. Prepare and mount new disk; first identify new disk
   fdisk -l
4. Partition new disk
   fdisk /dev/(newdisk)
   Make partition primary partition with type "83" file system type.
5. Create filesystem
   mkfs.ext4 /dev/(newpartition)
6. Mount new filesystem
   mkdir /mnt/(newpartitionmountpoint)
   mount /dev/(newpartition) /mnt/(newpartitionmountpoint)
7. Copy disk:
   /sbin/init 1 (drop to single user mode)
   rsync -avxHAX / /mnt/(newpartitionmountpoint)
8. Update FSTAB on newdisk
   blkid (note UUID of new partition)
   vi /mnt/(newpartitionmountpoint)/etc/fstab
   Replace existing UUID of / in FSTAB to new disk UUID
9. Configure grub and install to new disk boot loader:
   grub-mkconfig
   update-grub
   grub-install /dev/(newdisk)
10. Copy grub.cfg from old disk to new
    cp -ax /boot/grub/grub.cfg /mnt/(newpartitionmountpoint)/boot/grub/grub.cfg
11. Open grub.cfg on new disk and replace all UUIDs with new disk
    vi /mnt/(newpartitionmountpoint)/boot/grub/grub.cfg
    Replace all old UUIDs with the UUID of the new disk
12. Shut down computer
    shutdown
13. Physically move the new drive to the 1st drive location and remove old drive
14. Start computer and grub should present:
    

    error: no such device: xxxxxxxx-xxxx-xxxx-xxxx-xxxxxxxxxxxx
    
    GRUB rescue>

15. Manually boot new OS from grub; first identify the drive and partition of the boot files
    ls [to identify your drive and partition options]
    ls (hdx,p)/ [to identify which partition has the /boot folder]
16. Then, you can load the boot menu manually from the drive and partition you found above. Typically this would be (hd0,msdos1).
    set prefix="(hdx,p)/boot/grub"
    set root="(hdx,p)"
    insmod normal
    normal
17. Login to OS on new drive
18. Configure grub again
    fdisk -l (note dev of newdisk)
    grub-mkconfig
    update-grub
    grub-install /dev/newdisk

And that should be it!