Sorry it's a crap one. But what's the difference? (blush)
Well, I've run into trouble with this recently too. I've just looked it up in Fowler, and I'm only a little the wiser.
'on to' is older - 1581, than 'onto' - 1715
'On to' is two words for things like: 'Everybody is on to (i.e. wise to) that' or the 'We must walk on to Keswick' or 'Pass it on to your neighbour'. I think the principle is that here the 'on' is an adverb describing what the verb's doing more precisely, (walk on, as opposed to walk back) and the 'to' is a preposition, showing how the verb-adverb phrase then applies to the object (to Keswick, not past Keswick)
Onto is one word when it is a preposition showing how the verb applies to the object: 'We drove onto the beach' or 'I dropped a hat onto my head'
I would cheerfully write things like 'he threw it onto the roof' but I've had a copy editor change it to 'he threw it on to the roof', and to be honest, I should have but didn't stop to ask myself why until now. Even now I understand a bit more, there are clearly overlaps: 'I drove on to the beach' could mean where you headed towards after the party, or that you put the car on the sand. 'I drove onto the beach' clearly means the latter.
They do have quite distinct meanings, and different pronounciations as well - or they do when I say them.
'I drove on to the beach' means something like, I drove past the golf club, up the hill until I got to the beach.
'I drove onto the beach' means something like, I drove the car through the dunes and parked on the sand.
In the first, the vowel in 'on' is much longer.
There are some cases that clearly call for the use of 'onto' and others where 'on to' cannot be replaced.
'Zorro jumped from the balcony onto the back of his trusty steed.
'David stopped the car. The passengers knew he was very angry and they ceased shouting. David re-started the engine and drove on to the hotel.'