The SG20k is a pretty hard (tightly bonded) hone. A new diamond plate of 325 grit or thereabouts will flatten it very quickly, but it will slow the plate down some afterward due to diamond wear. An old diamond plate that's got worn diamonds will take a dog's age to flatten the SG20k. Personally I find the best approach for flattening very hard stones/hones is - as already mentioned by several fellows - to use loose grit. You can take the stone/hone down to flat pretty rapidly this way, then give it a quick scuff with your preferred grit diamond plate to smooth it.
Another point I'd like to make is that there is some misconception about the "negative space" not doing any honing that I've seen mentioned by several fellows that isn't really completely accurate and isn't being thought through very well. Using a coarse grit to lap a hard stone can be bad news in terms of the finish that will be imparted by the stone afterward.
The "negative space" implication is that the surface of the hone is quite flat but has furrows plowed in that surface that are lower than the mean average surface height. This isn't really the case - in reality there are those furrows but there are also pointed peaks created as well - which break off easily during honing and create an auto-slurry which creates a coarser surface finish on the item being honed. Eventually this wears down to that flatter surface with "negative space" furrows, but until it does it is to the detriment of the created surface finish. This is much worse on harder hones (they take longer to wear down) and it's far worse of an effect when new diamond plates are used for lapping - they create deeper furrows and higher peaks than worn-in plates.