The hard disc format is determined by the type of processor you have
in the machine: GUID format for Intel chips, MacOS Extended for PPC
chips. I have a G4 [PPC] chip, so both Ext. and Int. are formatted
with MacOS Extended.
The EXTERNAL has a different system, 10.4.11, under which LilyPond
2.12.2 functions just fine.
The INTERNAL has system 10.5.8, under which neither LilyPond 2.12.2 or
2.13.9 will work at all.