Eric Johnson
Preferences-sync and caching?
Yes default should be 128kB and would work well for most projects, but if the external SATA is not working as well as internal for some reason, you might need to raise it. But this would normally give you a dropout with drive icon in the Performance module of the Control Bar going red before the dropout.
Whatever you do, don't give priority to Background Processes. That was always bunk. Nothing that SONAR does or uses runs as a service. All you'd be doing is giving priority to Windows housekeeping operations and things like Bluetooth and WiFi over SONAR, which you obviously don't want to do.
You should Google and download the free LatencyMon or DPCLAT, and check your DPC Latency, just to be sure that isn't part of the problem. Lower is better, but anything under 300us (microsoconds) peak should be fine.