Sorry that you're having some issues, we're not aware of any issues with using 18i20s on Yosemite.
You seem to have found our article on 18i2096k & OSX, however presumably you're having the same issue across all sample rates?
Please can you download and install the latest beta version of Mix Control 1.9b10 from the below link and advise if you're still experiencing the issue: http://beta.focusrite.com/file.php?id=1192
Please can you also try disabling your Bluetooth and Airplay features to see if this makes a difference.
Let me know how you get on.
Simon // Focusrite Technical Support
That's quite unusual.
The 18i20 is effectively a plug and play device on Mac and we're not aware of any issues with Scarlett and Yosemite.
Try also deleting the following:
Mac HD -> System -> Library -> Extensions -> Delete FocusriteUSB2Audio.kext, FocusriteUSBAudio.kext & NovationUSBAudio.kext if they are present.
Once done, please navigate to:
Finder -> Go Menu (top of screen) -> hold 'ALT' on keyboard -> select newly appeared 'Library' option -> Preferences -> delete com.apple.audio.audiomidisetup.plist
Once done, navigate to:
Mac HD -> Library -> Preferences -> Audio -> delete both files.
Empty the trash and then restart your computer
Do you get the same issues when using the 18i20 across all the USB ports on your Mac? Try unplugging any other non-essential USB devices to see if this helps across each USB port.
Are you using the original USB cable? If so, try using a different USB cable that it under 1.5meters.
It would also be helpful to see if the same issue occurs if the unit is used with another computer.
Let me know how you get on.
Simon // Focusrite Technical Support
I saw around internet the some people solved their issue with too modern moderboards by getting the big brother of that card with 7 USB2 ports. And this little cheap card solved my crackling problem.