Problem changing disks in D2 (US version) on a PAL DC

[Dieubussy]

I've got a US Dreamcast console and the game D2 (NTSC-US). All disks work just fine, I've tested them when I played the game some years ago.

A friend of mine who owns a Pal DC has borrowed my game and was able to launch DISC 1 using a Utopia loader disk. However he said that whenever he needs to change disks, the console won't recognize the new GD-ROM - says it's not valid.

I don't think this is the sort of game that allows one to save in between disk changes - as that would be the solution I guess. Is there any way to change the disks?

Thanks in advance!