External drives · Drive formats
What is the hidden EFI partition on my external drive?
Short answer: It's a small, normally hidden system partition that GUID-formatted drives carry for starting up a computer. It uses very little space — leave it alone and don't delete or reformat it separately.
It's a small system partition that drives formatted with the GUID Partition Map scheme carry. It's used when starting up a computer and is normally hidden. It takes very little space; leave it alone, and don't delete or reformat it on its own.
If it appears in Finder or Disk Utility, it's usually because something mounted it on purpose; ejecting it is harmless. See why one drive can show as two.