It's actually going to require a combination of U1 firmware and an updated driver for the Mac, both of which are still a few months away.
The driver will enable you to write MXFs back to the disc. The firmware will enable you to expand the 'General' area up to the remaining capacity of the disc (currently it's locked at 500 MB). We'll have an announcement on the website when the driver is available, or you can feel free to check in with us in a month or two.
Curt Pair passed this along in a prior thread about archiving to disc:
A person in Sony's POSC recently sent me the following email: "GENERAL folder storage update is on the way but will first be released for the pdhd1500, then the U1, then possibly the F75 and other phase 2 machines. ETA is sketchy but it should happen within the next few months." (note: other phase two machines are F70/F30)
So now it's a waiting game. I think the Sony folks have created a great architecture that, once they start figuring out some of these features, is going to get even better. Sure, it's a bummer that the archiving end of it is running behind, but I remain optimistic.
Understand the frustration. I tried the EX convert to MXF with clip browser but can only access the 500mb 'general' folder with them. Yet with XDCam PDW-330/350 formatted discs I can back-up 1440 x 1080 35mbit MXF files with no problem and in doing so access the full extent of the 'clips' folder on the discs. I can archive finished 1440 x 1080 MXF edited files to disc no problem. It must be something to do with the PDW files being 1440 x 1080.
Hope they come out with the unlimited 'general' folder firmware/software sooner than later with the ability to back-up the 1920 x 1080 MXF files. Amazes me that this capability was around from the start with the PDW 'clip' folder files but wasn't implemented for EX with the release of the EX-1 when it was screamingly obvious that this was going to be an ideal EX-1/3 back-up/archive workflow.