Hey Kai!

 KR> It did and i removed the new disk to re-attach the old one. Then
 KR> no boot anymore.

I have the identical issue between nvme and sata.  On legacy it was never an issue and usually F11 ... or F12 ... on bootup would allow one to change from the default boot disk.  With UEFI it has been hit and miss but usually I can get sda to boot up if nvme fails to be seen which can and does happen.

Life is good,
Maurice

... Ne by¨ ¨‘t fele freond, se ¨e o¨rum facn hele¨.
    He who harbours treachery against another is not a faithful friend.
--- GNU bash, version 5.1.16(1)-release (x86_64-lilmikii-linux-gnu)