Thanks for the responses. I do have a good number of stills, but they are in RGB. And 55 min. of ProRes IS a large file, but it all is going into an empty 1TB external drive, so there is plenty of space there.
Now, this is a 55 min. doc that is subtitled. So, I'm thinking that the subtitling may make it render heavy.
I suspect that the problem is in the amount of rendering because I get the same message when I try to render the sequence before exporting it out onto a QT file.
So, this is where I go dark because I'm not really a computer techie. How is the rendering achieved? I thought that the system used HD space when it ran out of RAM.
My sequence is made up of 3 different segments from 3 different original sequence files. Would it help if I bring in all 3 sequences as nested files?
NO! DO NOT NEST!
Now...are any of these stills over 4000 pixels? If so...FCP will get that error. Not 100% of the time, but most. Gotta stay under 4000 pixels. FCP doesn't do more than 4K.
Render before you export. And render in small chunks, so you can narrow down where the trouble spot is. 5 min sections at a time. I had to do this for an online last week. A lot of stills over 4000 pixels, and film grain effects on clips causing issues.