> HDDs have a lifespan of approximately five years
For an active datacenter, no?
And speaking of active, active storage lends itself to monitoring and automated healing a lot better than tape archives.
Backblaze is fameous for using piles upon piles of consumer grade discs in their offering. Which is why their stats are so valuable to folks who are not running data centers, but might want a reliable drive for their NAS.
But really, it's mostly about scale. Tape media is dense in a way that spinning disk can't catch up to.
Visually, an LTO cartridge cannot have much above a half of the volume of a 3.5" HDD and it must be at least 3 or 4 times lighter.
Carrying a suitcase with twenty LTO cartridges is as easy as carrying any other typical suitcase, but carrying twenty HDDs would not be easy at all, due to the great volume and weight, and the volume in a real situation would be greatly increased by the need of adding some serious padding, in order to avoid catastrophic damage if the suitcase were accidentally dropped.
More importantly, the price per TB of tapes is much lower, and when you consider the ratio between the expected lifetimes, the price per TB and per year is much, much lower.
Of course, tape drives are extremely expensive, so you need to store at least a couple hundred TB to break even with the price of HDDs.
OTOH, HDDs have much better latency, as even a powered down hard-disk will power up and retrieve data faster than the time it takes to load a tape and read it. I can imagine a number of ways to partition a HDD pool in ways that the access patterns allow for most drives to be unpowered at any given time.
Most of the time the right answer is most of the storage in tape, with a couple of petabytes of disk in front of it to deal with hot data. Even in that circumstance you have to carefully manage the disk storage and filesystem or you wind up with client devices too new to access the supported fs version, another problem that basically does not exist with tape.
Can also be done with tapes, but you would require multiple tape drives (though if you can afford an automated library, thats probably nothing in comparison), additionally you can't find the failures until you take them out of "cold" storage.
If you can respond to the first failure when it happens, you can react before being hit by later failures hitting the duplicates. Downside is, since you have active components, failures are expected to be more common.