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Recapping:

External hard drive; 3TB Western Digital, p/n WDBWLG0030HBK. Connected to a windows 7 computer via a usb cable and also a power cable. Hard drive always spins up normally and sounds healthy; it does not make any clicking or grinding noises. Ruled out USB cable and USB port as problem ...

Yesterday while running all those commands and tests on it, including myreport.txt, it was showing this in Disk Management: Disk 1 Basic 2794.49 GB Online. (F:) 2794.49GB RAW Healthy (Primary Partition)

But today it's showing this in Disk Management: Disk 1 Basic 2794.49 GB Online. (F:) 2794.49 GB ...

Found it; thanks.

smartctl 7.2 2020-12-30 r5155 [x86_64-w64-mingw32-win7-sp1] (sf-7.2-1)
Copyright (C) 2002-20, Bruce Allen, Christian Franke, www.smartmontools.org

Smartctl open device: /dev/sdf [SAT] failed: \\.\PhysicalDrive5: Open failed, Error=2

What does this mean?

I stopped TestDisk and downloaded smartmontools v7.2-1 and attempted to run the smartctl command many times via the instructions at the link you provided, but so far none has worked. I have tried the following commands so far without success:

smartctl -a /dev/sdf > myreport.txt
smartctl -a /dev/hdf ...

The log file size is 825 GB as of this moment. My local HDD shows 25 GB of space left.

Two days ago my 3TB WD HDD was working perfectly; yesterday it failed. It shows up in Disk Management as RAW.

Disk Management shows this exactly: Disk 1 Basic 2794.49 GB Online. (F:) 2794.49GB RAW Healthy (Primary Partition)

When I attempted to run chkdsk, it was unable to execute.

I have been ...