Lightroom :: Why Does 4.4 Fail To Read Photos On Import

I insert an SDHC card into a card reader, LR starts, reads the photos and generates a preview so I can select which photos to import. I click on import and LR shows a progress bar that proceeds too quickly. When the progress bar completes LR informs me:
 
"The following files were not imported because they could not be read. (50)"
 
50 is the count of files that "could not be read".
 
1. LR read the files to generate the previews.

2. I can read the files with Explorer and copy them to folders.

3. I can then tell LR to add the folders that I copied to the files to and LR can read them just fine.
 
This behavior is consistent across multiple SD cards from different cameras with different formats of files (jpeg, nef, mov).
 
I am running Windows 7.1 64-bit with a quad core i7 and 12GB of memory.