Quick answer

Check the identifier with diskutil list, use sudo, confirm the disk uses the GUID partition scheme, and look at diskutil info to see if it is already mounted. If the volume is reported as damaged, run fsck_msdos on it. ESP Mounter is a quick alternative.

If sudo diskutil mount fails, or the EFI partition never shows up in Finder, work through this checklist from the top. Most problems are one of the first three.

Checklist

How to
  1. Check the identifier. Run diskutil list again and make sure you are using the identifier of the row whose TYPE is EFI, such as disk0s1. An EFI partition on a different drive has a different identifier. See how to find it.
  2. Use sudo. Run sudo diskutil mount disk0s1 and enter your administrator password.
  3. See whether it is already mounted. Run diskutil info disk0s1 and read the Mounted line. If it says Yes, the partition is already available at /Volumes/EFI.
  4. Confirm the disk has an EFI partition. Only disks that use the GUID partition scheme have one, and Apple Silicon Macs do not use an EFI partition on the internal drive.
  5. Repair the volume. If the error says the volume may be damaged, check it with sudo fsck_msdos -y /dev/disk0s1, replacing the identifier with yours, then try mounting again.
  6. Release it if it is busy. If you cannot unmount, close anything that is using it and try diskutil unmount force /Volumes/EFI.
diskutil list
diskutil info disk0s1
sudo diskutil mount disk0s1
sudo fsck_msdos -y /dev/disk0s1

A shortcut that skips the guesswork

ESP Mounter finds every EFI partition on internal, external and removable disks and mounts it with one click, so you do not have to match identifiers by hand. It also warns you before you touch a boot-critical disk. Once it is mounted, continue with editing your OpenCore config.plist.

Frequently asked questions

Why does diskutil say the volume failed to mount?
Usually the identifier points at the wrong partition, the volume needs repairing, or the disk has no EFI partition at all. Work through the checklist in order.
Why is there no EFI partition on my disk?
Only GUID-partitioned (GPT) disks have one, and Apple Silicon Macs do not use an EFI partition on the internal drive. A disk using Master Boot Record or Apple Partition Map will not show one.
Is fsck_msdos safe to run?
It repairs the FAT32 filesystem and can change files, so copy off anything you need first when you can, and run it only on the EFI partition identifier you have confirmed.
Is there a way to mount EFI without Terminal?
Yes. ESP Mounter mounts every EFI partition with one click and shows which disk each belongs to.