So you formatted your EFI Partition into a HFS+ Partition using diskutil eraseVolume "HFS+" "EFI" disk0s1 (replace with appropriate disk) and now you want to convert it back. Tried to Google without any success in figuring out how to fix this without backup/partition/restore? Well here is a way to do this... You will want to make sure that you have a backup just in case... but you already do that anyway right?

This will make the assumption that the disk that you formatted was disk0.

% sudo -s
# gpt -r show /dev/disk0
      start       size  index  contents
          0          1         PMBR
          1          1         Pri GPT header
          2         32         Pri GPT table
         34          6         
         40     409600      1  GPT part - 48465300-0000-11AA-AA11-00306543ECAC
     409640  547017448      2  GPT part - 52414944-0000-11AA-AA11-00306543ECAC
  547427088     262144      3  GPT part - 426F6F74-0000-11AA-AA11-00306543ECAC
  547689232       1264         
  547690496   77451264      4  GPT part - EBD0A0A2-B9E5-4433-87C0-68B6B72699C7
  625141760        655         
  625142415         32         Sec GPT table
  625142447          1         Sec GPT header 


We are interested in line "40     409600      1  GPT part - 48465300-0000-11AA-AA11-00306543ECAC". The GUID for Mac OS X, filesystem HFS+ is "48465300-0000-11AA-AA11-00306543ECAC". This is what we want to change back into an EFI filesystem which uses the GUID of "C12A7328-F81F-11D2-BA4B-00A0C93EC93B". So how do we fix this? You do have a backup right? If you do not, and you trash your data, do not blame me!


Unmount the disk:
# disktool -u /dev/disk0


Trash the HFS+ partition (disk0s1)
# gpt remove -i 1 /dev/disk0


Add the EFI partition
# gpt add -b 40 -s 409600 -i 1 -t "C12A7328-F81F-11D2-BA4B-00A0C93EC93B" /dev/disk0


That is all there is to it! Hope this works for you. I have done this more than once and it has worked every time!

Share this info but please give credit where credit is due: paige@projectsnow.net