In the sales I purchased a large Western Digital external HDD. I don't really trust hard disks anymore, but all other options are uneconomical or equally untrustworthy, so it's all I have for now. At least it's guaranteed. Anyway I face some troubles when backing up. I had to use gdisk to create the GPT partition instead of parted, for reasons I can't really fathom, but I'll probably stick with gdisk until further notice now.

After repartitioning the new drive the next task was to consolidate 2 generations worth of backup data onto it. I usually stick with rsync -aPv to mirror file trees. I copied all the data from both generations into subdirectories on the same disk. However, it also contains several complete Linux filesystems with archived copies of /sys, /proc, /run, and other data that I don't really care about.

rsync -aPv expands to rsync -rlptgoDPv. -D is not really wanted, though; it's an abbreviation for --devices and --specials, which we don't want. However, all other options we want. We want to be able to do all operations as a regular user. Although some files are sensitive, the backups live in a privileged space, so perhaps we don't care too much about security within the space itself. In this case we can do chmod -R o+rX /tree. This uses X for "special execute" which will make directories world-executable while not affecting the status of the execute bit for files. It will also make everything world-readable which obviously comes with heavy caveats.

We can add the -u or --update option to the rsync command, this will overwrite identically named files in the tree with newer versions from the source. Obviously this does have the potential to lose data, but it may be a reasonable trade-off to make the filesystem more manageable; YMMV. As we're not using --delete the target tree will essentially be an accretion of files; files that get moved will potentially create duplicates. We consider this an OK trade off relative to the dangers of using --delete.

You can use the --log-file=foo.log option to store all progress to a log file which you can examine afterward. You'll want to vet the transfer reasonably carefully to make sure everything completed and you're not deleting potentially valuable things.