RAID1 degraded on NSA325 v2 after file deletion

Mijzelf

Yep. That I/O error definitely is the cause sdb is offline. And I guess there was a comparable error during sync, causing the sync to stop.

The sync always stopped at 0.4%? That is about 1 minute? In that case it's probably not the power supply. If that is failing, it would stop almost directly when both disks are pulling their max power. And I think it's neither the SATA bus, as that doesn't know about 0.4%. So the disk itself remains.

if you can suggest if I can / should upgrade something.

I see you are using a rather old version of Tweaks. The latest contains a mitigation for a vulnerability which allows an attacker to take over the NAS when you only open a malicious webpage in the browser while being in the same LAN as the NAS. Scary. Yet I don't know if it's ever exploited.