OSX DiskUtility fails due to inability to unmount USB drive

On OSX 10.9.3 trying to format some USB drives but the Mac Disk Utility failed when trying to reformat or repartition with " error unable to dismount " and consequently fails. This seemed a bit hit and miss which USB drives it effected but the fix was to temporarily disable Spotlight indexing - why it was indexing these particular drives it was not clear. Note after re-enabling indexing OSX will reindex all disks which will take some time.

To disable Spotlight in Terminal use command

       sudo mdutil -a -i off

After using DiskUtility re-enable indexing in Terminal with

       sudo mdutil -a -i on