Turned out not to be that easy as it sounded...
Situation has worsened: As soon as the new HDD is attached to any controller, ArcaOS shows the ArcaOS logo at boot, remains there for 20 minutes... regardless if booted from this very HDD, or another HDD or a maintenance stick.
In case of the install/maintenace stick, after the logo goes away, every step tages atleast 10 minutes until the graphics build up, windows shop up, etcetc takes forever to complete... until I am able to do a thing with it.
As soon as I remove the faulty HDD it's as "fast" as usual.
So.. I took out the faulty HDD, installed a new fresh ArcaOS on the new drive.. but now ... how to copy the data over, when I cannot use the maintenance boot thing of ArcaOS? - I could of course boot into Linux via stick, which has no problems with the faulty drive, and simply copy over the data... but won't that result in weird rights for all the files? Or might that work out?
If so, I'll do that... the maintenance thing does not work for me.