This is a strange one, as I have replaced many a drive, but this time i lost a whole 4TB of data when upgrading from a 4TB to a 16TB drive, the drive showing as only 181 GB used.
Unraid Version: 6.12.4
1. I removed the drive
2. Insert new drive
3. Started array
4. Clicked format
So i guessing above was correct unless i missed a step
Luckily i was replacing the drive so i still have the data to restore it, but wanted to raise this and see if i missed a step or something has changed for next drive