4 x 2TB drives in RAID 10 configuration.
I tried with drives 1, 2 & 3 in and 4 out - perfect power up, NAS (with correct name) visible in Qfinder and visible to laptop File Explorer.
All folders and files intact.
I had good data back-up.
Drive 4 has had a couple of Read/Write errors recently and I had bought a spare drive to change over at this month-end off-site back up process.
All good (the power outage must have finished drive 4 for good !!)