@Cre

I found a posting somewhere that explains why you'll get an error "Invalid Argument" when mount_hfs /dev/disk#s# /Volumes/EFIthat is because by default EFI partition is a FAT32 and with that command you're trying to mount it as HFS. If you follow NoSmokingBandit's guide to the letter you need to download his InstallPack as well and run sudo ./Make-EFI-Boot-Now-v3.sh instruction is included in the zip.

Ciao!