Seems the most common relationship to this error is using Windows on a separate partition. Here is my problem. I have a 1 TB external disk that was used half for Time Machine and half for general storage, partition called Mac Swap. In order to make a more elegant back up, I replaced my Time Machine with a full 2 TB external disk. This allowed me to delete the old half (just rendered obsolete) and extend the Mac Swap half to a full 1 TB. But that is where I got stuck. I can't extend the partition, Mac Swap.
Command+S only works on the boot disk, right? /sbin /fsck -fy won't solve this. But on the bright side, it seemed to repair a few minor errors on my boot disk.