I've just used MIDIOX to pull the patches out of my Poly-800 II; in so doing I discovered that the 4th byte in the string should be 0C not 01 as it is said to be for the Mark I and EX. FYR the whole thing is

F0 42 21 0C 10 F7

If you get it wrong the machine gently tells you

F0 42 21 0C 20 F7

Hope this will help some folks.mn

One more observation for me and the other person with a P800II: apparently it is possible via MIDI to push the sequencer parms. (they're global, remember) out of legal range... whenever you load patchbanks from someone else, go right to para. mode and look up parms. 85 and 86 , clock and repeat. If there is a letter or high number, or it can be heiroglyphics, hold the DOWN button until the display scrolls to a 1 or 2; if you don't the results are highly unpredictable up to and including crashing the machine.mn