EFI with Multiple Disks / Partitions

[chiechi]

I struggled with this for quite some time, so I thought I'd post it for anyone else who finds themselves in this situation.

My disk layout:
Solid -> SSD with macOS partition and a Scratch partition for temporary files
Data -> SSD with home directory
Backup -> HDD with partitions for backups of Solid and Data

After installing High Sierra, Solid looks like this:

/dev/disk3 (synthesized):
   #:                       TYPE NAME                    SIZE       IDENTIFIER
   0:      APFS Container Scheme -                      +240.2 GB   disk3
                                 Physical Store disk2s2
   1:                APFS Volume Solid                   120.5 GB   disk3s1
   2:                APFS Volume Preboot                 19.0 MB    disk3s2
   3:                APFS Volume Recovery                520.8 MB   disk3s3
   4:                APFS Volume VM                      3.2 GB     disk3s4

No EFI partition!

Scratch now looks like this:

/dev/disk2 (internal, physical):
   #:                       TYPE NAME                    SIZE       IDENTIFIER
   0:      GUID_partition_scheme                        *480.1 GB   disk2
   1:                        EFI EFI                     209.7 MB   disk2s1
   2:                 Apple_APFS Container disk3         240.2 GB   disk2s2
   3:                  Apple_HFS Scratch                 239.5 GB   disk2s3

Thus, to mount the EFI partition for Solid (disk3) I have to mount /dev/disk2s1
I am sure this was obvious to some, but it took me a while to figure out that Solid (my main macOS partition) is now a virtual disk mounted from a physical disk that is now named after the Scratch partition. Thus, to find the correct EFI partition, look for "Physical Store diskNsN" on your boot disk where diskNsN will point to the physical disk on which the EFI partition resides.