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.
