In meantime if look at my pm as I remembered that Uelpenich hasd a problem once with booting
He got the problem after fiddling with the disk. initially the install via fvdw-sl console worked as expected
Uelpenich.
I was afraid that my 2Big1 became a brick. It stopped doing anything, it even didn't boot from the fvdwsl console.
Then I did the factory reset ceremony, and I prepared a second disk with fvdw firmware V10.
I put this disk (3TByte formatted as 2TByte dos partition table) to drive slot 2 (right hand slot), together with the first (2TByte) disk in slot 1 (left hand side).
Surprise: the system booted and works. I get an interesting mixture of the new V10 system in slot 2 and my data in slot 1.
Note that he mentioned that it didn't even boot anymore from the fvdw-sl console.
So apparently there can happen something that makes the boot process not work.
So maybe you suggestion to try the nwsp2 system disk is a good idea
One remark: The nwsp2 is a better device then a 2big1, cpu is 2x faster + nwsp2 has 4 times the amount of RAM
But can the disk of the nwsp2 in your 2big1, just write the normal 2big1 kernel to sda6 (so not the standalone) to the disk of the nwsp2 and put that in your 2big1. It should boot but some modifications might be required to get all features working