What John said. Also, I wouldn’t rely on HDD as my only archival format. Certainly a USB HDD is about as future-proof as one can imagine for an electrical interface (do use USB; no guarantee SATA will still be around… some laptops are already using PCIe based flash modules).
And hard drives do last a long time. It used to be about five years of active operation, today, maybe ten or more. But just sitting there, there’s no guarantee. It might be still good in a 100 years. Or it might be mechanically compromised: corrosion in the bearings, stiction due to hardened lubricants, etc.
I have all kinds of stuff backed up on HDD. The critical stuff is also on optical disc. In particular, good Blu-rays are likely to outlast just about anything else: they don’t scratch as easily as DVDs, they use phase-change rather than organic dyes as a recording medium, etc. Of course, we don’t know about format rot… but it’s extremely likely Blu-ray sticks around for the consumer move to 4K video (BD-XL already exists, and is already large enough for 4K), so you can ask the same question in 10 years.
And that’s the real key to archival — upgrading your archives. What seemed like a good idea in 1991 (backup to tape) might have been a bad move in 2001, worse yet in 2011. The good news is, of course, that everything you had archived back in 1991 probably fits on a single BD-R. If in 10-15 years we’re decided that those cheap 10TB Memristor-based USB 4.0 drives most people use for their 8K video collections (all downloaded, since everyone but me had unlimited gigabit Internet to the home), then those archives will all transport easily enough.
-Dave