I know this thread is quite old, but I think I've figured out how to do this. Since the solution isn't listed here, I'm hoping I'm not posting something that is now obvious.
I'm using version 0.8.6c (wxWidgets interface). First, I go to File, then Wizard, then choose Transcode/Save, and Next.
I choose Select a stream, and browse to it. I check the Partial Extract Enable box, then in the From field, type the starting point in SECONDS where I want the partial extract to begin, and then the end time in seconds as well.
Here's the strange part. It only seems to work if the From point is 4,290 seconds or less. If I type 4,300 in From, it invariably begins transcoding from the beginning of the file.
At the moment, I'm testing a multi-transcode approach for a file that is 3:55:30 long. I can easily extract just the first two hours, but since I can't tell it to start at 7,200 (two hours) and extract the rest, I'm going to tell it to start the extract at 4,290, and go to the end. Then, I'll do the same thing again on that resulting file until I've progressively trimmed it down to being the final 1:55:30 of the original file. Not very elegant, but it should allow me to get this big file on two discs.
Mark
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www.northernlight.net
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