I was experiencing some very bad dropped frames for a while, with no answers from the forum...I did some experimenting today, and am please with the results I have....
I am using SD,NTSC, 720x480, so I set everything to 59.94i, turned on "Blend" for camera and video playback, ticked Interlaced recording, but am unable to keep the settings for my USB grabber to 59.94i, so I have it set to 29.97, and for 1 hour of recording, I have 3 dropped frames on internal HD, and 3 for the same period on a USB external HD, until I opened 'My Computer', then I got 16 dropped frames.....so, I will run another test, giving vMix the highest priority available to see what happens....
Update: I have also discovered that setting a high buffer for recording also adds to dropped frames....a buffer of 10 - 15 works ok, anything higher caused dropped frames with me...setting vMix to a high priority, causes PC problems, seems to be causing high rate of dropped frames........strange, but true......