If you want to work with growing MXF files from just:in multi with FCPX, please do the following:
- check if you have the latest just:in multi version installed
- check if you have the latest FCPX version installed
- check the Import Settings within FCPX and make sure that "Leave files in place" is selected
- check if your MXF capture preset has "Update headers" disabled
If you are still experience problems, do not hesitate to contact us.