Incremental backup would not complete - ATI 2014

Hello all.  I posted a few days ago when I was having a different problem.  That one seemed to fix itself, but this one isn't.

I had been using ATIH 2010 for several years with no problems, but I started getting an error with it, so I figured the problem might fix itself if I upgraded to a newer version.  That's why I upgraded to ATI 2014 not long ago.  Unfortunately, the same problem is back.   See the attached screenshot.

It is saying it is failing to read data from disk "4".  First off, I do not know which disk it is talking about.   I assume it must mean my C drive, because the backup is setup to backup my C drive to my U drive.

I ran CHKDSK on my C drive, and after taking forever, it came back and said there was nothing wrong.   Just to be safe I ran CHKDSK on my U drive, but no errors were found on that one, either.

I don't understand what is wrong.  Can anyone help?  My incremental backup is scheduled to happen overnight, and obviously if this error is happening, it isn't working.  It worked for years with ATIH 2010, and I am not sure what might have happened to suddenly make it stop working.

Oh, and I'm on Win 7 64-bit, in case that makes any difference.

EDIT: I should also probably mention that my C drive is an SSD.  It is a Crucial MX100 512GB.

Thanks,

Matt