Waryle

@Waryle Thanks for update, and the fine description. Yeah, BTRFS is a problem i did also hit the "wall" and did also go through with Ext4, i havent got any downtime yet.

That was very kind of you, thank you!
I have finally found the problem after hours of looking up for documentation and retries. I had the idea to take a PC monitor and plug the RockPro64 on it to see what kind of error it could throw, and work from there :
Uboot seem to boot only on the first SATA drive of the list : I had a SATA card with 4 ports, I plugged my SDD in the port 1 and my two other drives in the 3 and 4. Uboot does not like BTRFS. Once I have plugged the SSD on the port 1, I had a initramfs error, so I went again in armbian-config, used the "Boot from SPI - System on SATA" option, and this time I chosed Ext4 instead of BTRFS. It's working now. I could have found a way to put only the /boot on an Ext4 partition and the system on BTRFS, but I have lost too much time and I'm gettting lazy, so I'll just leave it that way, Ext4 is perfectly fine anyway.
 
Thanks for you help !

Sorry, the forum limits me to 1 comment every once in a while, I couldn't answer earlier.
 
@soerenderfor
I did use a Samsung SD Card ( https://www.amazon.com/Samsung-Class-Micro-Adapter-MB-MC32GA/dp/B0749KG1JK/ref=sr_1_3?dchild=1&keywords=samsung%2Bsd%2B32gb&qid=1607026358&sr=8-3&th=1 ).
I did flash the u-boot-flash-spi-rockpro64.img.xz using those instructions : https://github.com/sigmaris/u-boot/wiki/Flashing-U-Boot-to-SPI
I even waited an extra 5 minutes afterwards before rebooting just to be sure.
 
I also tried Sigmaris' 2020.10 uboot version, and asked for help there : https://forum.pine64.org/showthread.php?tid=8685&pid=84734#pid84734
 
Just to be sure : you are currently booting from SPI directly to a SATA drive, not from EMMC, right?