On Fri, Aug 29, 2025 at 10:55:22 +0100, mick.crane wrote:
For the purpose of backing up 3 ~200Gb disks, with Debian operating
systems
on them, I wondered if I can put them all on one 1Tb disk and be able
to
copy them back.
Well, yes, you could.
Not really understanding how dd works wondered if a bootable disk can
be
copied to a partition on another disk for the purpose of backup.
Or if the disks should be copied to ISOs. I've not done that before.
Forget ISO (by which I assume you mean an ISO 9660 file system image).
It has no relevance here.
If you "dd" each of the 200 GB disks to a file on the 1 TB disk, then
yes, in theory, you could at some future point "dd" that image file
back to a different 200 GB disk and boot it. This constitutes a valid
backup strategy.
However, you should be aware of the shortcomings of this backup
strategy. You have to copy *every* byte of the disk -- even the
ones that aren't being used for anything (gaps between partitions,
empty regions of partitions). When you want to update your backup
next week, or next month, there's no way to just save the differences.
You have to re-copy *every* byte again.
Given that the three 200 GB disks total 0.6 TB of space (roughly),
you can't even store two copies of each disk image on your 1 TB
backup disk. You only have room for one copy of each disk, plus a
bit of working space. E.g. when you want to update the backup of
disk A, you could copy it to a new file, and then delete the old
copy of disk A once you're certain the new file is complete. Then
do the same for disk B, and so on.
Each of these copy operations is going to take a LONG time, because
you're copying the whole disk, every time.
A more traditional backup strategy would be to copy the *files* from
all of the partitions on all of the 200 GB disks. The disadvantage of
this is that you aren't backing up the boot area (MBR or the UEFI
equivalent), so the backups are not bootable. If you have to recover
from a complete disaster, you would have to install a minimal Debian
system on a new 200 GB disk to get it into a bootable state, and then
copy the files over from your backup. Getting it to actually boot
might take a bit of tweaking.
However, the advantages are legion. When you update the backup, you
don't have to re-copy every single file -- only the ones that have
changed. rsync and similar tools are designed to make this easy
and efficient. And if you want to get fancy, you can keep multiple
versioned/dated copies of each source system. There are backup suites
that build on top of rsync, giving you a way to store many backups
without needing to store duplicate copies of the unchanged files.
Something to think about.