This also happens with japanese units. All effects from updated IOP modules which are loaded from the HDD instead of the ROM (these get patched by the mod while being loaded.)
As a curiosity, you will notice that newer versions of software like DVD Region X, Gameshark/Action Replay are now added to the banned list of the PS2 ROM, being unusable while the HDD is connected to the machine.
SCPH-10000_GH-001 SCPH-15000_GH-003 SCPH-18000_GH-008 SCPH-30001_GH-005 SCPH-30000_GH-016(V4) SCPH-30001_GH-010(V4)
2xSCPH-10190, 2xSCPH-10350, 2xSCPH-10280
"**** j0 hackers!"
-Sjeep (As seen on TOXIC OS ELF...)