this is almost what I do,
I use 2 different USB hard drives that I keep at my work's office (encrypted)
every weekend, or whenever I remember or when I am back from vacation and a lot of new photos exist,
I bring one of these 2 to home and run the backup
On each drive there are more than one backup tasks, even if my total data on the NAS are only about 1 TB
Some shares are archives from my job, they don't change even in 6 months, so these tasks do not run so often
other shares change constantly, and I included these on the (backup) tasks I run every week
there is no reason to have only one backup task for the whole NAS,
additionally for some of my shares, I want different retention rules, eg for my PC disk images I don't want to keep backup versions of 1 year or more old,
so these go to a different backup task
each time I bring a USB HDD to home for the Hyperbackup I run the tasks I want, not every existing task
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HyperBackup is a very versatile application for backing up the NAS,
and is one of the two* reasons I haven't yet moved to FreeNAS
(even if Synology's lack of support for Linux with desktops apps is pushing me to do so)
* (the other one is PhotoStation)