All I can say is, whatever data storage method you use, make sure there are multiple backups in geographically diverse locations. It would be a shame to lose so many months worth of work to a simple data loss.

Make daily backups of your work onto USB flash keys as you progress along with your job. At the end of the project, make your final master backups on more than one copy of more than one media. For example, two or three portable hard drives, two or three stacks of burned DVDs, and maybe even two or three sets of usb flash keys.

Then collate each group of diverse media (group=drive+DVDs+flash) and geographically separate them (1 at office 1 at home 1 in safe deposit).