It's been  a rule of thumb for me to make sure the drive that poser is on has enough free space to store the movie uncompressed; I've had graphics apps in the past that needed the space, as they rendered out each individual frame then compressed them into the finished product only at the end of the process. I have no idea if Poser or the Quicktime compressor work this way.
Also, if you are working with dynamics, both cloths & hair make pretty big temp files.

However, the last Apple product I touched was a new Lisa.