3.5 Problem with Project being on USBDrive (on Mac)

This hasn't happened before 3.5 and it only is happening on the Mac. Pretty much with the project being on a USB drive, every time I minimize Unity and then go back to it, the "Importing Assets" loading bar shows up and it reimports every single asset - every time. Then at the end of the reimporting the "The open scene has been modified externally" and it's asking me if I want to reload; if I reload it, it reverts to the last save, and nothing happens if I ignore it. I don't see how minimizing Unity and then unminimizing would be a cause for it need to reload every asset and then say the project has been modified externally.

 

Go to Edit > Preferences > untick Auto Refresh.

That should fix your problem, but I don't think Unity projects were ever intended to work off USB drives, they do an awful lot of I/O operations which the transfer speed of USB isn't great for.