It seems like there might be a compatibility issue between the SSD and the second MBP. Have you checked if the second MBP meets the minimum requirements for the SSD or if there are any firmware updates available for either the SSD or the MBP? You could also try resetting the SMC (System Management Controller) and the NVRAM (non-volatile random-access memory) on the second MBP to see if that resolves the issue. If none of these steps work, it may be worth contacting the manufacturer of the SSD or Apple Support for further assistance.
Discussion
Thanks for trying to help, but it's not worth it. I'm just ranting. The order is completely random. It happens with two different SSDs and they work/ed on both devices. On Linux, never once a problem.
Basically doing the last data transfer. These SSDs are not going to touch these MBPs ever again. They will have a great live on my linux machines.
I gave up. Even when I managed to get the ssd mounted, it randomly unmounted and was a pain to get re-mounted. I decided to connect the ssd to a linux machine and sync the data with rsync over ethernet. This has been running for hours now without issues. But I had to buy an usb-c to rj45 dongle. Actually, it was quite an adventure to find one 😆
The problem is really interesting. linux systems are really much better but they don't get the attention they deserve
The funny thing is, these SSDs worked last year without problems. Also interesting, the SSDs are noticeably more stable on the Intel MBP than on the Silicon MBP. I think it’s related to the current macOS version.