No NDA's for me. My betas are all friends, I pay them in beef jerky, kind words, and promises of a t-shirt if I ever sell enough books to have a few t-shirts made. In some cases I have the pleasure of providing my own rudimentary attempts at being beta for their work.
For storage, I use laptop, email, and dropbox as well as the odd flash drive. And my betas have copies in their emails should I ever have a total crapocalypse.