What Fergler said....
It's ok to run the app and plugins from usb, reaper will load everything into ram anyway... but you want to avoid streaming any samples, unless your 2nd drive and connection are very fast, like usb3.
Edit: my ssd C drive is only 128gb. It's 100x faster than all my other drives put together, so I have to run everything off of it.
Sadly, it's the sample and loop data that benefits the most from residing on the ssd... thankfully it's only 60gb (relatively small), and my reaper+plugins folder is about 2gb, leaving me about 40-50gb for recording.
I really need a bigger ssd drive