[Rich Breen]

Darryl suggestions are all good - I'm immediately suspicious of Backblaze; copy operations in the background will often throw errors in Pro Tools - even with cache on.

I really think this is something external to Pro Tools causing the problem (but of course, no guarantees). Run Activity Monitor, be sure "All Processes" is selected in the view menu, and try sorting by CPU and/or Disk - keep this running and visible while you're working and see what pops up.