It does really depend on how much data you want to store, but if you've got a lot of it…

Tape.

Obviously extreme prosumer, but for long-term archival of lots of data, LTO tape wins in several ways:

- Discs just aren't actually that high capacity relative to modern HDD capacities. BD XL maxes out at 128 GB, while there are now 30 TB HDDs readily available. That's 240 discs per HDD. Modern LTO tapes store 12-18 TB, or 2-3 tapes per HDD.

- Anything flash-based is a bad choice for long-term storage. SSDs are very fast, but also (relatively) expensive at 15-20¢/GB. Reputable SD cards are in the same neighborhood. Despite the OP redditor's results here, flash is only expected to retain data for 5-10 years.

- Tape is the absolute lowest cost-per-GB you can find of any storage medium. At the moment, LTO 8/9 tape can be had on Amazon for ½¢/GB. Compare with BD-R at 2¢/GB, or BR-R XL M-disc at 15¢/GB. HDDs (spinning rust) are 2-3¢/GB.

- Consider also write speed. LTO can write 300+ MB/s. BD 16x maxes out around 68 MB/s.

- Manufacturers rate tapes for 30 years sitting on a shelf, and it wouldn't be surprising if they still read after 50 years¹. Plain BD-R lasts 5-20 years. M-disc is the interesting outlier, rated 100-1000 years.

Of course, the biggest problem with tape is the drives. While the media is dirt cheap, the drives are crazy expensive. It looks like you can pick up a used LTO-6 drive (2.5 TB tapes) on ebay for around $500. A brand new LTO-9 drive (18 TB tapes) will be $4000-5000.

In terms of breakeven points, a used LTO-6 drive + tapes beats plain BD after about 25 TB. Because of the cost of M-discs, they stop making sense after 1-2 TB. Purely on cost, a brand new LTO-9 drive + tapes doesn't beat used LTO-6 + tapes until about 800 TB (LTO-9 tape is ½¢/GB while LTO-6 tape is 1¢/GB), but there's definitely a point in there where the larger capacity of LTO-9 makes dealing with the physical media a whole lot easier.

So if you're looking for long-term storage for your photo album, a M-disc BD XL is probably a good choice. If you only have a few hundred GB of data, a couple discs + burner can be had for $300 or so, and you can be pretty sure your mom could manage to read the disc if necessary.

But if you're looking to back up your 100 TB homelab NAS, discs are not really feasible. You'll have to spend the next month swapping discs every 25 minutes², and then deal with your new thousand disc collection. Here's where a used LTO-6 drive makes a lot of sense. This is a real sweet spot if you can find a decent drive; all-in you'd spend about $1500 to back up your 100 TB.

This is what I do to backup my NAS — found an old LTO-6 drive and got a bunch of tapes. The drive plugs in to a SAS port (you might need a HBA PCI card, $50), and that's pretty much it. Linux has the drivers built in; it will show up as /dev/st0 and you can just point tar³ at it.

Finally, just to compare with cloud options, storing that 100 TB in AWS Glacier Deep Archive would run you slightly over $100/mo, so you're ahead with your own tapes after little over a year. Oh and don't forget to set aside an extra $8000 for data transfer fees should you ever actually want to retrieve your data lol.

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¹ eg the Unix v4 tape that was recently found and successfully read after 52 years — https://news.ycombinator.com/item?id=45840321

² Or get a disc-swapping robot, but those run $4000-5000, at which point… you're better off with a brand new tape drive.

³ Thus using the Tape ARchiver program for its original purpose. Use -M to span tapes, tar will prompt you to swap.