Backup to multiple disks

I need some advice on how to set up my backup.
I have an internal data disk (6 TB) that I would like to back up regularly to two backup disks (3 TB each). Mirroring is enough, I don't need a history. The backup disks are installed in hot-swap trays and are stored elsewhere when not in use.

I would prefer to just slide the trays in to start an automatic backup. When it's done, I get an email and just pull out the disks.
I was looking for a ready-made solution, but I couldn't find one. The USB backup plugin can only be used for backups to one single hard disk. So I'll probably have to write a script.

Problem 1)
I don't want to manually split the internal hard disk data between the two external hard disks. I was thinking about using the Unionfilesystems plugin to merge the external disks to one file system. Are there better alternatives (raid)?

Problem 2)
Hot plug. On hardware level, the hot-plug already works, I can plug in and use the disks during operation. How can the hard drives be safely removed? Just pulling it out is certainly not a good idea neither for a raid nor for a unionfs. So how do I unmount them?

Problem 3)
The simplest part: copying the data, I would use rsync for this.

At the moment the 6TB disk is just filled up half, but this will change. Most data will be a single share containing lots of subfolders which i dont want to split up.

Maybe I could somehow use symlinks to spread those subfolders into 2 backup folders and use USB backup to backup the subfolders. But I would have to watch the folder size permanently...

lol, sometimes the simplest solution is also the most obvious.

However, this might lead to another problem: The 6 TB disk is encrypted with luks (I didn't mention that before ) and I unlock it manually via web interface after reboot.
If I encrypt the 2x3 TB disks with luks and merge them with unionfs, wouldn't that be almost the same szenario as with hot-swapping?

I didn't have time to dig in that direction on the weekend, but I'll look further into the unionfs/mergerfs... There must be a way to mount/unmount the overlay filesystem.

I'm worry about the short period of time when one disk is unlocked and the other not. That's the same szenario as if one tray is inserted/running and the other not. Mergerfs has to deal with this flawlessly.
I ran some quick tests yesterday:
I created and mounted two partitions sdc1 and sdc2. Then i created a overlay filesystem (fs1) from the mounts and copied some data on it. Afterwards I unmounted and remounted sdc1 and/or sdc2 serveral times and watched what happend to fs1. Aaaaand..... it happened what I expected only in the best case. Exactly all data of the mounted partition(s) were visible in the second it was mounted. I didn't check for any errors though.

Additionally I could use "umount" to unmount fs1. Not sure if thats enought for a secure removal, but the readme says mergerfs does not use any caching.

So maybe I was worried about nothing.... The shiny world of linux is new to me

I'll try to make up a selfmade backup skript.

On last thought on using 2x3 TB as internal disks with mergerfs... there seems to be some issued with certain database software i.e. plex. I didn't understand it completely, but may it's better not to do that.

I'm making progress...

1) for unmouting the overlay filesystem the command

fusermount -u /mountpoint

can be used (see http://manpages.ubuntu.com/man…ty/man1/fusermount.1.html). I don't know if makes any difference to using "umount". I guess it doesn't make any difference, since I can hardly find any information about unmouting mergerfs or fuse filesystems.

2) A script for the whole process of mounting and backup can be started via "udev" rule. See: https://brokkr.net/2017/06/12/…iling-and-how-i-fixed-it/