More El Capitan woes, it seems, because the new Disk Utility app in El Capitan appears not to create Windows-Compatible USB sticks — even if the all-important option for Master Boot Record (MBR) and FAT32 formatting is selected when you click to Erase or Partition the stick.
It’s a bug and the fix is to close Disk Utility, open a Terminal window (it’s in the Utilities folder of the Applications list), and then paste in the following:
defaults write com.apple.DiskUtility advanced-image-options 1
Then erase or partition the stick in the usual way using Disk Utility, after which you should find it’s now compatible with Windows.
This bug will probably be fixed in an upcoming release (well, you’d think it would be), so you might want to make a mental note to revert this change back to default values in future. The following will do the trick:
defaults delete com.apple.DiskUtility advanced-image-options