“DSCF0019.RAF” could not be used because the original file could not be found.
Why is this occuring on my iMac with Lightroom 5?
Is there something I've done that causes some images to go missing?
If you moved or renamed or deleted the photo (or the folder that contains it) outside of Lightroom, then you will have this problem.
Why is this occuring
The usual reason is because you moved the file to another location using Finder. No way for LR to know that unless you tell it by clicking on the exclamation mark icon in the grid view and then clicking "Locate".