I've never had a GS550, but of the four or five 1000's and the one 750 I've owned, I've only ever had to replace one starter motor, and sadly, it was a result of my own ineptitude.
I'd just installed a Yoshimura 1085cc ki, and a pair of Yoshi cams, on my GS1000S and decided to do a speed test on my favorite cop-free stretch of road. I wanged it out to a little over 140 MPH before I ran out of road, then on my way home, pulled into a garage to fill the tank.
After I gassed it up, I thumbed the starter, but nothing happened. Ooops. I push started it, and it fired up straight away, so I rode it home and parked it. Next morning I went to investigate, and eventually pulled the starter. (no carb removal required on a GS1000)
It was locked solid. When I took the end caps off it, the internal components came out in small pieces, reminiscent of shrapnel. Double Oops. I couldn't buy a good used starter anywhere, so I ended up buying another complete GS1000S for parts.
I showed it to my Cuz, a factory trained Suzuki mechanic, and he told me he'd only seen one other as bad as mine, and surmised that I'd inadvertantly touched the starter motor when at "full warp". Cheers, Terry.