I was doing some rsync backups and noticed a primary drive was having some hardware sector issues. It was 2TB with a lot of data.

I was thinking maybe I should separate the OS+DATA.
Put the OS on a small 64GB SSD and the DATA on its own magentic 2TB drive. Then use rsync again as my backup tool.

Will this prevent the DATA drive from failing ? Of course not.
But it will save me hours of time rebuilding the server and
hours of putting all the DATA back to where it was before.

I suppose the SSD drive could fail as well, but without moving parts seem to be more reliable.

Maybe I could get of the SSD and boot from a USB.

Another option is to run a FreeNAS with mirrored 4TB.
Or any NAS with a mirror.

What would you recommend as a distro to put on a USB ?

TIA

If you have a backup drive with sufficient space, you can clone the entire drive - including boot sector and partition UUID(s), and put it in another computer so it can be kept in sync regularly using rsync. (The options I use for such things are "rsync -vaxAX /. /mnt/nfsshare/".) Still, you really want the destination computer to NOT be booting off the cloned drive. Conflicts with the same IP address, weirdness with system updates, conflicting daemons, and so on. With a cloned drive, then you can swap in the backup drive in case of failure and you're good to go.

But still, you're idea will work. An SSD failure is significantly less likely than a spinning hard drive failure. I wouldn't be too enthusiastic about USB reliability, though. I use USB stick installs day-to-day, and my experience has been that they are more likely to fail than a spinning hard drive.

Either way, I've had good success with plain old Debian. A regular desktop workstation install will fit comfortably on an 8GB USB thumbstick, and if you're careful about avoiding bloat it will fit comfortably on a 4GB USB thumbstick. But I'd use a thumbstick only for less important purposes. For example, booting the backup server off a USB thumbstick can make a lot of sense. If there's a failure, no big deal.

Besides the possible reliability issue with a USB thumbstick, it also probably performs poorly compared to even a spinning hard drive. At least, that has been my experience-admittedly with USB2.0, but there are perceptible brief hangups which can't be excused just by USB2.0 bandwidth limitations.