As sminlal indicated, USE two storage devices. IE two External HDDs, or a External HDD and a DVD drive. Blu-ray has higher capacity, but also higher cost per disk. For cost a Blu-ray SL is about 22 gigs vs 4.7 Gigs for DVD SL.
For External HDDs. Understand, they do fail. Also for long term storage the "Cells" (magnetic domains) can corrupt adjacent Cells when stored for long period of time also the bearings can create a problem when stored over a long period. Solution is to periodically power on and rewrite the data.
For DVD disk.
(A) DO NOT use permant markers to write on. The acid in them can eat thru to the recording media - Bye-Bye Disk, they do sell a marking pen safe for DVDs. Not a fan of "Labels" but have used my ink-jet to print on printable version.
(B) Store in Cool DRY location. High relative humidy decreases how long the disk will retain the data.
(C) Do not write above the rated speed, I often write to the disk at 1/2 rated speed for disk I plan on keeping longer than 5 years.
(D) Use single layer disk, not the dual layer (DL) ones.
(E) Buy QUALITY Dvds, not the cheapest one you can find. Most writers will so a preference for a given brand/lot. I normally runs a program to measure the quality of the burn on two samples from the package.