Sounds like you got a challenge ahead of you! Can you restore from backup?
From a Sia perspective, you would first shrink the storage from 2TB to 50GB or so, then remove the old drive and replace it with the new drive.
Remember that Sia must always see the storage in the same location as before (i.e. drive letter [on Windows] and path).
However given what you're describing, I wouldn't be surprised if even the shrinking process failed, as Sia needs to write to the file on the failing drive in order to shrink them. In this case, reverting to a backup, or being able to scrub the bits from the failed drive using whatever disk recovery software, are your only options.
Good luck!
