i've also read that they can coexist, but I've never used roxio so I can't say for sure.
just keep a close eye on your system. whenever you add or remove software make note of what was done and WHEN. this will help you track down any conflicts or changes for future troubleshooting. it's also a good idea to make a habit of manually creating system restore points before installing or uninstalling anything that you're leery about.
sounds like you're on the right track though! have nero, roxio, AND sonic was definitely overkill.
not sure if roxio has a packet writing software, but if it does, i'd get rid of that as well. also, get rid of Nero InCD (this is assuming you don't need the packet writing junk) no use in having stuff on your computer that could cause problems if it's not necessary.
now that you've uninstalled sonic, are both of your drives back to normal?