Mount /dev/md0 and write to it?

Zitat von vladd

boot my box from an usb drive with clonezilla(omv is not running at that moment)

There is a plugin to allow you back up your omv configuration.

Zitat von vladd

Another use-case: When I'm running docker containers on the SSD, but want to have the data on the harddisks (which are raid configured). Is this possible from the viewpoint of the raid?

Create an smb share on your raid then point/store your docker configs too...look at the guides section plenty of info in there.

Zitat von vladd

I would

boot my box from an usb drive with clonezilla(omv is not running at that moment)

clone the ssd to somewere on /dev/md0

If you have omv-extras installed, there is an option to download clonezilla and to tell omv to boot for one time into clonezille. This option is not available for ARM.
I use this option to clone the system SSD to a data drive. However, I do not use RAID.

You can also use the backup plugin, as geaves mentions, but be aware that restore needs some knowledge. And it is a backup of a live system. Using clonezilla is probably the most reliable solution.