I think the dual filesystem approach sounds good (at least for my personal target of faster CF boot, play mp3s).
Keep all the boot stuff on the CF and allow the natural order of things (according to Roger) to use the HD until there's not much free space (which sounds like a reason to get a lower capacity CF too maybe even just a gig).

I guess I can achieve this by making the system boot with one (CF) drive only and then following the upgrade process to add a second drive.

I don't think this needs much (well, it doesn't *need* any) modification to Hijack either does it? Just clever setup.

Hmm, so what Hijack configurations would I like?

Spin up 2nd drive (asynchronously) at boot : Y/N
Y=I tend to play HD stored music and want faster boot.
N=I tend to use AUX/FM and want to avoid spinup unless the player sends a media request

Am I understanding?

PS I know I'm merely speculating - maybe you can consider the design implications of all these new fantasies

PPS Damned bbs timeout - I had to take a call whilst writing this - when I submitted it it said "The form you have submitted is no longer valid". Ended up using ethereal to capture the re-POST from the browser cache - fun though.