Couldn't Modify Partition Map using external disk 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. 

Please help.
I found my silly error. The solution: in Disk Utility, I chose specifically the partition that wasn't able to increase in size, then I ran the verification. It turned up problems so I ran the repair command. Then I was able to increase the size of the partition. Mt external hard disk, Mac Swap, is now only one partition. 

To be clear about my error, in case you're reading this for help. In Disk Utility, there is always both an icon for the disk as a whole and below it immediately there will be all of its partitions. If you run the verification on the hard disk as a whole, then it may not catch what's wrong on your partition.