So, even though the destination for the footage being compressed is an external drive connected to my machine, hard disk space is still being drained from the local drive. Why is this? I can't find where compressor is sending these 'ghost files,' as they're not named anything close to what the actual files are called. Nor can I find any backup or local drive usage options in the compressor's preferences.
Apple keeps many of the inner workings of Compressor hidden but I wouldn't doubt that it's creating a temp file.
Encoders such as Episode allow the user to set the temp folder in preferences. The temp file can vary with settings and codec but two pass encodes and certain process filters create very large temp files.
I can't say if Compressor is behaving the exact same way but I don't doubt it's something like a growing temp file.