Always backup you hard drive. (so you do not have to worry about it failing)
Acronis True Image WD Edition Software
Macrium Reflect Free
See 2TB WD Black second HDD slows to a crawl in Windows 10
Its always good to have spare hard drive when testing for hard drive problems.
I would backup the current hard drive and restore the data to a new hard drive.
Then test the hard drive with Data Lifeguard Diagnostic for DOS (current version 5.27)
Follow the instructions to create a bootable USB.
Turn off you computer and remove the sata power connector from all the hard drives.
Connect the hard drive that you think may be defective.
Turn on the computer.
Set your bios to boot from a USB device.
Select WRITE ZEROS TO DRIVE and do a full erase.
It should take about 2.5 hrs depending on the computer (2 to 2.5 hours per tera byte).
It not only erases the hard drive but it also checks to see if it is working properly.
if the drive is defective it will come back with an error code that WD support wants for a RMA.
If the hard drive passes format the hard drive.
Then goto http://hdd.userbenchmark.com and find the read and write speeds for your hard drive.
Benchmark the hard drive.
The speeds from crystalmark should be close to the ones at userbenchmark.
If they are not
Userbenchmark Read 100MBs Write 100MBs.
CrystalDisk____Read 100MBs Write 50MBs.
CrystalDisk____Read 50MBs Write 100MBs.
Then the hard disk may slowly be dying.