Xbox-Linux seems to support LBA-48 (where Xbox kernel only supports LBA-28, limiting it to 137GB), so it seems that it's not a 'chipset' issue (as was previously thought)

I've got some great news - I have the MS Xbox KERNEL reading/writing an LBA48 drive using the LBA48 commands (meaning: drives > 137GB will be possible)!
The KERNEL recognizes that it's an LBA48 drive at startup time, and sets the drive size accordingly (I'm using a 200GB drive, and it's reading the size correctly).
The KERNEL read/write functions are now using LBA48 commands, and they work!

Now, I've not tried reading/writing past the LBA28 limit on the disk, but there's no reason it won't work - the only possible issue was whether/not the LBA48 commands / command-structure would work.

I've got to clean up the error checking, and finalize a couple more routines. Then, I'll write a test app that creates a bunch of unique files on the F: partition, and goes back/verifies they all are still unique (to confirm nothing 'wraps' and overwrites back to the beginning).

I've also got a simple 'partition table' mechanism, so there can be up to 8 partitions on the drive. The main reason for this is to allow for other operating systems (like Linux) to co-exist with the Xbox (usually, the Xbox uses up the entire remainder of the drive for F:, leaving nothing for Linux).

Since I'm not sure if/when mod bios developers will incorporate my changes (and whether/not they'll incorporate ALL of my changes - I'm really concerned that the 'partition table' stuff may get ignored since it really only helps other OS's like Linux), I'm also in contact with the developer of Xbtool, and it's possible this will be a selectable option that can be applied as a 'patch' to any bios - just like the various other color/etc patches he's already got.

I hope to have this all finished up in the next couple of days (depends on my 'real' job, and how busy I am there). I just wanted to give an update, since I think people are interested in this.