Hi Mark,
The Yellow line is related to processing, meaning the the processor should be good enough to keep up with the footage.
If, as you described, you are working with a single drive, that drive will not keep up with the data rate of the Targas. In that case rendering (via preview) to a format of a lower bit rate (as in the iFrame codec) will solve that issue. The other solution is investing in a RAID array.
I hope that answers your questions.
Cheers,
Vince Becquiot
Kaptis Studios
San Francisco – Bay Area