That was
just a test that the file was still accessible.
If you could not open it, Calibre would have no better luck
BTW that is dated 9 Sep 15 and you were using Calibre 2.35
How many books? FAT has limits on how many files in the drive root (\ )

I just use author_sort-(1st letter) folders there, so I never hit the limit
You probably should reformat as Kovid suggests to FAT32 . you will lose the reading progress status as you will need to reload