This drove me nuts for months.

I start my Windows 7 music PC to do some updates or transfer some files.
I turn on my Scarlett 18i20.
I start Sonar, insert a soft synth, turn on record, and play for about three minutes. Everything is fine until I stop and do something like rewind or turn off record, especially if I do those things immediately after recording. Then Windows freezes and stops accepting input from keyboard or mouse. The only thing I can do is hit the restart button on the box. Event Viewer shows nothing relevant.

Hint: This happens fairly consistently, but only once a day.

Solution:

If I turn on the Scarlett before the PC, the freeze doesn't happen. I don't remember reading in the Scarlett manual that I had to do this.

Mitch I.

Weird, but it's great you figured it out.  It's the kind of strange issue most folks never find, then end up blaming Sonar.
 
You do have the latest firmware/drivers for the Scarlett?  You should let Focusrite know of your problem/workaround.  They might be able to fix it... Worst case, they don't, but it's in their database, and someone else might still benefit from your troubleshooting.

My first reaction was to blame Sonar. I couldn't decide whether to contact support or post in the main forum. I'm glad I did more investigating.
 
I just got the Scarlett this year, and they haven't updated the drivers since then. But I agree, letting them know might be worthwhile.
 
Apart from this strangeness, it's been a great unit.
 
Mitch I.

JonD,
 
I took your advice and contacted Focusrite support. They answered promptly and sent me a driver that solved the problem.
 
Mitch

Mitch_I
JonD,
 
I took your advice and contacted Focusrite support. They answered promptly and sent me a driver that solved the problem.
 
Mitch

Hey, that's good news.  Hopefully, the fix/driver update will be officially released soon.    
 
Thanks for letting us know.

seems like most of the time,
when i read these kinds of threads,
the problems are almost always simple driver issues.
 
or the device is broke.
 

This is so interesting because i find that nothing ever works if it's not on already at startup haha. My MIDI Keyboard, my audio interface, my printer. That's just always been my experience with everything USB except for like flash drives and hard drives.

Mitch I want your Coffee Cup!

For the coffee cup, I made a screenprint of the About box (without my serial number) and sent it to Vistaprint. Then I put it on my synth keyboard and photographed it. Sometimes I pretend I got the cup from Cakewalk.