Limitations of RN51600

After running 24/7 for a couple of years now, I experienced my first disk problem with a WDC WD4000FYYZ-01ULB0 4TB in slot 3 of RN516. The logs started showing this message a couple of weeks ago:

Detected increasing reallocated sector count: [1234] on disk 3 (Internal) [WDC WD4000FYYZ-01UL1B0, WD-WCC130771614] 34 times in the past 30 days. This condition often indicates an impending failure. Please be prepared to replace this disk to maintain data redundancy.

So I ordered a replacement disk (and another spare disk) of exactly the same make and model: WDC WD4000FYYZ-01ULB0 4TB. Assuming that hot swap was possible and the disk would automatically be rebuild, I took the new disk out of the packaging and immediatly swapped the faulty disk without stopping the RN516. When removing the faulty disk, the volume was indicated as "degraded". When insering the new disk, resync started, but lastet less than 30 seconds, after which the volume showed up as "Rebuild, but still degraded" and depicted with red color in volume display. After reboot, all disks in the volume were marked red and I wasn't able to get it working again. The only option left was to do a Factory reset. Daily backups have been taken from the data, so it should be OK, allthough the work on setting up the apps and web server will be cumbersome. After factory reset and creating the volume again, all disks show up blue with green leds in the volume depiction.

Is there anything I could have done differently, to have the disk swap run smoothly and have the volume automatically rebuild?