error message" "disk copy was changed since you last opened or saved it."

[#1]

I am getting an error message I never have seen before, after making some changes in a file and trying to save it:  "The disk copy of [file] was changed since you last opened or saved it.  Do you want to save anyway?"  What is the "disk copy"?  I only have one version of this file, and it lives on an external hard drive, where I edit it.  I use Windows 10.  What does this message mean and how should I proceed?  

[#2]

I am getting an error message I never have seen before, after making some changes in a file and trying to save it:  "The disk copy of [file] was changed since you last opened or saved it.  Do you want to save anyway?"  What is the "disk copy"?  I only have one version of this file, and it lives on an external hard drive, where I edit it.  I use Windows 10.  What does this message mean and how should I proceed?  

[#3]

The mesage means what it says:  something outside of Photoshop has modified your file on disk since the time that Photoshop saved the file.

We don't know what might be changing them -- just that the OS returns a different modification time for the file than what it was when Photoshop saved the file.

[#4]

Thanks, I saw this reply in a previous thread.  But I don't understand it.  I edited the file after saving it previously.  Does this mean that if I save my file now, the changes I have made since last saving it won't be saved?  And/or that there are other chnages that will be saved (that I did not make)?  

[#5]

With all due respect, this answer, copied verbatim from a post dated 2011, raises more questions that it answers.  How can something "outside Photoshop" be making changes to the file?  Without knowing a great deal about operating systems, how is one to understand how something like this could happen - and what one should do about it?  Specifically - what has happened (are there edits that have been made to the file other than those made by the user) and what is the best practice to preserve, or restore, the integrity of the file?