Steenamaroo
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Then, when export stereo files from Reaper and disconnect the Focusrite, it plays back at the wrong pitch on the Mac.
What happens if you do your recording and exporting, quit Reaper, then play back the wav or mp3 through Finder/itunes/Whatever through the focusrite?
If it sounds fine, that points to the focusrite as being the problem.
Focusrite have their own control software on MacOS, don't they? Check in there and make sure it's not overriding sample rate.