windows 10 - Hard disk bit rot?

One bit on my 2 TB hard disk occasionally changes from a 0 to a 1 or vice versa. This seems to be an occasional read error.

I found this out because I have a program which runs at startup, computes SHA1 hashes of all my data files, and reports any that are different from hash values saved the last time.

I happened to have an extra copy of the affected file, so I am able to perform a bitwise comparison. The file is a JPEG image file and I can view it without any reported problem (so the bit must be in the RGB data values rather than any crucial image metadata or header part).

My disk's SMART info suggests there is no problem.

C:> wmic
wmic:root\cli>diskdrive get status
Status
OK
OK

The disk is an ST2000DM001-1ER164 in device manager. All partitions show as "Healthy" in Disk Management.

This is what my startup file-checker says:

Filecheck report
Some previously "inactive" files have been modified!
C:\Users\RGB\Pictures\From-Phone\Images\IMG_20160719_090140630.jpg

I can check it's logfiles to see the SHA1 hash values for the two copies of the image:

C:> findstr 140630 filecheck.dat
2016-07-19      EMzHG9bZUqA1OkuiouZoN+mD8X4=    C:\Users\RGB\Pictures\From-Phone\Images\IMG_20160719_090140630.jpg
2016-07-19      DhbuPVUu6A4Eo7BIkQww17iCakk=    C:\Users\RGB\Pictures\2016\2016-07\2016-07-19\IMG_20160719_090140630.jpg

I can do a binary comparison to see what changed

C:> cd \Users\RGB\Pictures
C:> fc /b From-Phone\Images\IMG_20160719_090140630.jpg 2016\2016-07\2016-07-19\IMG_20160719_090140630.jpg
Comparing files [...]
0013B232: 40 00

That's a one bit difference. It doesn't look like crypto malware, maybe HD is going bad?

The next day

Filecheck report
Some previously "inactive" files have been modified!
C:\Users\RGB\Pictures\From-Phone\Images\IMG_20160719_090140630.jpg

Now both files are the same

C:> findstr 140630 filecheck.dat
2016-07-19      DhbuPVUu6A4Eo7BIkQww17iCakk=    C:\Users\RGB\Pictures\2016\2016-07\2016-07-19\IMG_20160719_090140630.jpg
2016-07-19      DhbuPVUu6A4Eo7BIkQww17iCakk=    C:\Users\RGB\Pictures\From-Phone\Images\IMG_20160719_090140630.jpg

I have both networked and offline backups of my data. I have created a Windows 10 system recovery drive on a USB flash stick.

What can I do to assess whether I need to replace the hard disk urgently?