I haven’t totally traced down the problems, but I have had two OSX portable USB drives stop wanted to mount.  I had formatted both drives as Mac OS Extended (Journaled).

They just stopped wanting to mount and OSX’s Disk Utility wouldn’t repair the disks to a state where they would mount.

I didn’t loose data because I purchased Stellar Volume Optimizer, which also fixes disks.  I got the data off the disks, reformatted the disk as simple Mac Drives (the default in Disk Utility) and put it back.  When the program fixed the drive it found extra folders used in formatting.

I suspect a bug in 10.9.4 and using Journalling on USB drives, but couldn’t find any comments on the web.