The “master” drive is a Lacie Rugged 4TB portable drive, connected via USB (3? I think). I tried moving the footage to a Samsung SSD, also connected via USB.
I just tried it on my new laptop with the SSD, connected via USB3 – the fastest I’ve got – and still having the issue.
Most recent versions of all … High Sierra 10.13.5, and PP 12.1.1.
The folder with the proxies are on the same drive as the source footage, but everything was moved (consolidating drives for a second editor) which triggered the problem.
In Modify Clip, I’ve set the audio tracks to 2, and selected channel 9 & 10 (where the good audio is), but when inserting a clip into the timeline, the scratch audio on Channel 1&2 appears.