As we handle machines remotely it is often useful to have a secondary boot partition that can be used from grub.

Basically, create a similar sized boot partition on a different disk and copy the running one over with:

parted -a optimal /dev/sdb
(parted) p
Model: NVMe CT4000P3SSD8 (scsi)
Disk /dev/sdb: 4001GB
Sector size (logical/physical): 512B/512B
Partition Table: gpt
Disk Flags:

Number  Start   End     Size    File system  Name  Flags
 1      32.0GB  4001GB  3969GB  ext4         bulk

(parted) rm 1
mklabel gpt
mkpart fat23 1 1GB
set 1 esp on
align-check optimal 1
mkpart ext4 1GB 32GB
mkpart swap 32GB 48GB
set 2 boot on # other flags are raid, swap, lvm
set 3 swap on
mkpart scratch 48GB 512GB
mkpart ceph 512GB -1

We also took the opportunity to create a new scratch partition (for moving things around) and a ceph partition (for testing). Resulting in

Number  Start   End     Size    File system  Name     Flags
 1      1049kB  1000MB  999MB                fat23    boot, esp
 2      1000MB  24.0GB  23.0GB               ext4     boot, esp
 3      24.0GB  32.0GB  8001MB               swap     swap
 4      32.0GB  512GB   480GB   ext4         scratch
 5      512GB   4001GB  3489GB               ceph

Now we have the drive ready we can copy the existing boot partitions and make sure you don't get it wrong and the target partitiong is larger. Here the original boot disk is /dev/sda (894Gb). We copy that to the new disk /dev/sdb (3.64Tb)

root@tux05:/home/wrk# dd if=/dev/sda1 of=/dev/sdb1
root@tux05:/home/wrk# dd if=/dev/sda2 of=/dev/sdb2

Next, test mount the dirs and reboot. You make want to run e2fsck and resize2fs on the new partitions (or their equivalent if you use xfs or something).