i haven't seen a pic of this in the 2500 but if it's anything like the one stereoroid showed pics of online back in the day it's most likely a diagnostic jack
which is kind of cool. i'd love to see whatever plugs into it. i'd love to OWN whatever plugs into it. you could learn so much about your machine.
i wonder if that's how they code the os's (or DONT code the os's)
otherwise, they'd have to create the code on the pc and then try it on the mpc over and over since the usb is not a master...
who knows? cool stuff though. it'd be even cooler if it was some upgradeable plug for something in the future. i remember when we first saw the HD tray (not adaptor) inside the mpc1000, everyone was like...what?
A diagnostic jack was everyone's best guess when a thread on the same subject was started over 1 year ago. But thinking about it further...I can't recall older MPCs having a similar jack.
...even if it can be used for diagnostic purposes, i think it is used to flash the os at the factory... many companies do it that way - the way the user gets offered for upgrades might differ from that...
the possibility to connect a daughter board is not very likely, because there is not enough room for it...