I am using Lightroom 5.2 (tested with MAC OSX 10.8.5; 10.9 and WIN 7/64)
All images are stored on an external drive an all smart previews are built.
If the external drive is unmounted and I try to email photos I get the error:
Can't send email because some selected photos are missing.
Exporting the photos to my local hard drive is working so the photos are not really missing.