NAS fails to boot with disk 1 engaged

[5AMsan]

Hi all,

I just get my 5Big2 back from repair (dead cap), sent after I noticed it was no more on the network.
Now, using a serial connection, I see boot failing probably because HDD is dead (see logs below).
How am I supposed to get the system back, while keeping datas on HDD2 & 3 (it was a RAID5 array)?

-- Edit --
Trying to get fvdw-sl console to load stand alone kernel, it just fails to connect nor interrupt the boot sequence.
Serial console is giving feedback on ongoing boot sequence (also attached below).

Boot fails
https://pastebin.com/ibGM97km

Boot with no drive
https://pastebin.com/L9srzxeV

Any link welcome, thank you all