Thank you for your reply!
But sadly no, that’s actually not the same thing. To quote from How to use the Team CoreELEC ceemmc tool
Option Backup:
This option can be used to create a backup of the current CoreELEC installation on the internal eMMC.
Also, it doesn’t work, because it wants to write to internal storage where there isn’t enough space (and it doesn’t reach the SD card anyways).
Which makes sense in the context, as ceemmc seems to think CE is booted from SD card, so it writes to /storage. But really this mountpoint isn’t on the SD card but on internal.
If I understand the how-to correctly, booted from eMMC I should get:
[Existing CoreELEC installation on eMMC]
[1] Make a copy of current CoreELEC installation on eMMC to current used SD or USB device
Make a copy of all CoreELEC data on flash and/or storage
partition on eMMC to the current used SD or USB device
Hmmm