I can answer that: If I recall correctly (it's been a while since I've had my hands on either one), the s550 and the s650 are the only "pro" PSR's (not entry level like the PSRE series) that doesn't use the modern interface (modern as in the PSR9000/2000 through the Tyros 5 and PSRs975 used the same interface). The s550 and s650 us an OS very similar to the previous generation PSRe (like the PSRe463) which means: you can't copy and paste styles or just read files from the flash drive like you can with all the other modern Pro PSR's: you had to first load a style into the user style slot (like the way Korg PA is setup, but instead of hundreds of slots to load into, you only get 5, I think? It depended on the model as well, some using this interface only let you save 1 user style at a time, some let you save up to 5; I'm pretty sure the psrs550 had 3 user style slots)
The even crazier part, is that we take for granted we can copy internal style files onto a usb for the modern (i.e. since the early 2000's) arrangers; but by using the older OS on these 2 models, you can't. You *could* edit a preset style, and after modifying, save that to USB. But this is insanely slow compared to doing a single copy and paste command.
Having said all that, I hope somebody did go through the bother of copying, editing and saving the on board styles. And no, the PSR550 and s550 are very, very different.
And probably not surprisingly, those are also the only to PSR-S series I can think of that didn't have proper line outputs (just a combo headphone/out jack)
Mark
