Rebuild Volume Taking Very Long Time

I have had an EX2100 4TB configured in RAID 1 for a few years and had to replace one of the two drives.

When I first got the NAS and configured it it took a day or so from memory, the NAS is telling me that is will take 228,648 minutes (158 days) to rebuild!!

Can anyone tell me if this is correct or do I have a serious issue where it simply wont rebuild?
Is there any way of checking the integrity of the process or speeding it up??

Any assistance would be much appropriated, thanks in advance.