So, I need to archive a TON of HD video. So far, I've been archiving video as H.264 files, but now that I'm 17TB into my available 21TB on my NAS, I'm looking for a more efficient method. Is there a better format to store these files? I've looked into H.265 as a possibly codec a bit.
h.265 would be a good answer since you are backing up a lot of HD video. From an archivist point of view, duplicating footage to a NAS is not technically archiving footage - just having a backup copy. Plus, if you are looking to repurpose the footage later, h.264 may be too lossy to be used as resolutions go up in the future (4K and beyond - just like digibeta can repurposed but VHS... isn't). It's more of a delivery codec than a mastering or archive codec like ProRes can be or JPEG2000.
If this is footage that is going to sit a long time until/unless it's needed, might I suggest Amazon Glacier storage or LTO as a better archival medium? Both are long-term storage options that are less likely to die on you (the more drives you have spinning, the less time you have statistically between drive failures even if they don't take your storage down). However, if you're accessing it frequently then NAS is a great solution.