After each day of shooting, copy the AVCHD file to a computer's external drive.
Copying the file to a computer's external drive more often (each scene) is preferred, but we understand when that gets tedious, especially over the course of a year. You can rename that AVCHD file with that day's date... Try it with non-critical video... capture 4 different start/stop sequences, copy the AVCHD file to the computer and rename the file that was copied to the computer... After the file is copied to the computer (and hopefully backed up on yet another drive, format the card in the camcorder using the camcorder's "format memory" menu selection.
Why do this so often? It is possible the flash memory card gets a problem. Its database becomes corrupted, the camcorder is stolen - with the memory card in it... whatever... If you use the same memory card in that camcorder for a month or year of video capture, that just becomes problematic.
You don't want the single card to fail without having copied that file anywhere. And even if the file was copied to one drive, you don't want *that* drive to fail, either. The idea is to have the important files in at least two places. And reformatting the flash memory for the camcorder takes the database failure and corrupting the flash memory card out of the equation.
Because a memory card can fail when you least want it to fail, best to have a couple of smaller cards rather than one big one.
The reason to copy the AVCHD files to an external drive has more to do with computer response time... Most "modern" computers use virtual memory even when they have a decent amount of RAM (minimum 8 gig physical RAM, more is better). Virtual memory is a reserved part of the start-up hard drive that augments physical RAM. It takes more CPU cycles to deal with the operating system + the video editor + the input/output of the video project files from the start-up drive... if the video project files are stored externally, they are read from the external, not the start-up drive. This offloads a decent amount of the effort from the start-up drive. If you are fortunate enough to have a SSD startup, then the performance impact is not so much...
How I would do it:
Create folder on external drive for the project. Name it whatever you call the project. In this folder, create a folder called G25 video. You may want others like "Stills", "Releases", "Permits", "Agreement" (for grip, crafts, audio, lighting, rentals, etc...) For the AVCHD files:
1) Capture video.
2) Copy to the external drive(s)
3) save card 1, insert card 2 to camcorder. Repeat steps Step 1, Step 2... probably through 4-5 cards. This should give you enough time to copy to the hard drive - and the back-up drive.
4) Format the cards when the video copied from them is on at least 2 external hard drives. If you never use cards 2-5 because you are doing this daily (or more often), great. That means you still have cards available if the card fails.
Use of a 2-drive RAID-1 system where the two physical hard drives in a single case are mirrored counts as having 2 drives. I use a 4-slot OWC cabinet that lets me have 2 pairs of mirrored drives. When you copy a file from the camcorder, it is written to 1 pair of mirrored drives at the same time. If one of these drives fails, the other drive still has the file. Pull out the failed drive and replace it with a working drive the same size as the one it replaced. The RAID-1 intelligence will copy all the data from the remaining drive to the new one and make sure all is synchronized.