ReadyNAS Ultra 4 disk swap not recognised

I have two ReadyNAS models, an NV+ v2 and an Ultra 4 which both contained 4 x2Tb Seagate drives.

Three weeks ago I replaced one drive on the NV+ v2 with a 4Tb WD Red drive and all is well.

Yesterday I replaced one drive on the Ultra 4 with another 4Tb WD Red drive and seem to have issues with the disc change being recognised. The ReadyNAS was ready to use again after a few hours and the log shows I replaced a drive and the RAID sync process was successful. I appear to have the same disc capacity and space remaining which is what I expected, however RAIDar still indicates the old Seagate drive details when I hover over channel 1 (with a green indicator) whilst Frontview indicates I have the old Seagate drive in disk 1 but it has a dead status.

All my data appears to be available and shows the correct space remaining when I use Finder on my MAC.

Is there anything I can do, bearing in mind a plan to replace all the reaming 2Tb Seagate drives with 4Tb WD Red drives over the next few weeks?

Should I be concerned about my data?

Any suggestions or help would be greatly appreciated.

Thank you.