C: Primary Boot HD: WD_BLACK SN770 2TB (2Tbyte NVME)
D: RAID1 (data only) SATA HDs:
- #1 *WDC WD4000FYYZ-05UL1B0 (4 Tbyte)* <--- ***BAD HD***
- #2 ST4000NM0035-1V4107 (4 Tbyte)

Not Assigned a Drive Letter: TEAM TM8FP6128G (Used to dual-boot into Linux Mint)

I built this PC 2 years ago, and set-up two 4-Gbyte HDs as a RAID 1, and now the software says the RAID is "Critical".
The research I've done says I need to replace the defective HD with a good one, BUT

In BIOS I turned off the RAID and selected AHCI, and then ran Victoria on both HDs. Victoria finds 16 uncorrectable blocks on the WDC 4 Tbyte HD,
but I want to know that it's 100% certain that the HD cannot be repaired and put back into use, before replacing it.

Also, I've never replaced a bad HD in a RAID1, and am afraid I'll do something and destroy the data that is on the good HD, so before doing
anything else, I'm here to find out what I should do.

I wonder if the common-sense 1st step is to simply indicate that the "bad" HD is a "spare" (whatever the RAIDXpert2 language is), and just let the
RAID software try to rebuild the array. Maybe the 16 uncorrectable blocks can be remapped? And again, I'm worried about doing something that
gets the data on the good HD overwritten.

Thanks in advance.

Why did you run a high intensive I/O program on data you define as critical?

If the data is truly critical, treat it as such.

Create a 1:1 image / clone of the healthy (Seagate) 4TB drive using a program like OpenSuperClone. Then rebuild the array to a new drive by assigning the drive you replaced (WD) as a hot spare in the BIOS.