Quick answer

Open Terminal and run diskutil list. The EFI partition is the row with TYPE EFI, and its IDENTIFIER (for example disk0s1) is the value you pass to diskutil mount. ESP Mounter shows the same partitions without Terminal.

Before you can mount an EFI partition you need its identifier, the short name such as disk0s1 that macOS uses for that partition. Every command that touches it, including diskutil mount, needs this value, and mounting the wrong one is an easy mistake when several drives are connected.

Find it with diskutil list

How to
  1. Open Terminal and run diskutil list.
  2. Scroll through the output. Each physical disk starts with a heading such as /dev/disk0 (internal, physical): or (external, physical).
  3. Under each disk, find the row whose TYPE is EFI.
  4. Read the last column, IDENTIFIER. That is the value you need.
/dev/disk0 (internal, physical):
   #:                       TYPE NAME                    SIZE       IDENTIFIER
   0:      GUID_partition_scheme                        *500.1 GB   disk0
   1:                        EFI EFI                     209.7 MB   disk0s1
   2:                 Apple_APFS Container disk1         499.9 GB   disk0s2

In this example the EFI partition is disk0s1. The numbers will differ on your Mac.

Narrow it down when you have several drives

Or let ESP Mounter do it

ESP Mounter lists every EFI partition it finds on internal, external and removable disks, shows the filesystems on each disk, and marks the disk you booted from. Pick one and click Mount, with no identifiers to read.

Once you have the identifier, follow how to mount the EFI partition on a Mac.

Frequently asked questions

What does disk0s1 mean?
disk0 is the physical disk and s1 is the first slice (partition) on it. The EFI partition is usually s1 on a GUID-partitioned disk.
Why do I see more than one EFI partition?
Each GUID-partitioned disk has its own EFI partition. A Mac with an internal drive and two external drives can show three. Match the parent disk by size and name before you mount one.
Can I find the EFI partition without Terminal?
Yes. ESP Mounter lists every EFI partition it finds, shows what is on each disk, and flags your active boot disk.