And even if you could split it … I think the Prime could not access them as two different “sources”.
As for that amount of music (even in WAV-format that is just an incredible amount of tracks to have bought, even through a pool), I have my opinion on that, but to each his own. With the new streaming options, there really is not much reason anymore to have much more than a core collection (say 750-1500 tracks) that can handle any “normal” gig and perhaps a cross-collection of possible request tracks. In mp3-320-format (no reason to have those “play once in a blue moon”-tracks in flac, you’d be done with about 500GB worth of tracks (around 50.000).
That said, your question is not about that, but about technical aspects of partitioning. Clearly GPT helps break that MBR barrier, but MBR is still the compatible gold standard. It’s changing slowly, but the main limiting factor here is actually the bios. To boot from a GPT-partioned disk, you’d need an UEFI bios. More and more hardware offers that. How that is implemented in other, task-specific hardware like the P4 is another question. Linux (or whatever flavor is used in the P4) basically supports GPT as well. So, I am guessing it will all come down to the bios-support.
Being able to read a disk (as you apparently can after you go passed the error message) is one thing, the main question is can you also write to it.
Again, personally I think a 2TB limit is fine. I doubt audio-files will ever grow much. 24-bit/192KHz has been around since forever in recording environments and still 99,9% of humanity is still on a maximum of 16/44.1 (CD-quality), which quite frankly is more than adequate for all but the most high-end audiophile environments. Heck, lots of people just listen to crap-quality youtube or free Spotify tracks on their 10 dollar earphones.
That said, it’s up to anybody to want more than 200.000 tracks on their hard disk.
I assume that the folks at Denon will look at the demand (how many people want/actually need more than 2TB disk sizes). If it’s high enough I am sure they’ll make it work (technically it’s probably already possible). If not, then you may be out of luck … or have to use a second (external) SSD/HD for your purpose. God knows the P4 has enough USB-ports
As always just my three cents worth.