I did a bit of searching and found a reddit threat that looks like this one but posted 6 months ago.

The only answer that looked somewhat correct was “Because they always have a spare in their trunks.” posted by u/A_Nice_Shrubbery777

Reddit thread for those interested: https://old.reddit.com/r/mildlyinfuriating/comments/1e1qbg9/a_joke_with_no_answer/

There is a much cheaper way that doesn’t use hard drives. It uses magnetic tapes, LTO-9 tapes specifically.

Each LTO-9 tape cassette can hold up to 45TB of data (compression is used to store it on the raw 18TB).

An LTO-9 tape drive can cost $10,000. Assuming you get the full 45TB per tape, you’ll need 2223 LTO-9 tape cassettes to store 100PB. Assuming you buy in bulk, you can get each tape cassette for $150 which puts you at $333,450 for the tapes.

Since the tapes don’t use power when not in use, this concludes the total cost. None of this accounts for storing all 2223 tapes or maintenance to ensure data is still intact on them but this comes out to $343,450 in total to store 100PB using magnetic tapes. While the cost is much cheaper, it’s much harder to access the data as it’s not immediately available since you have to fish out the drive you need and plop it into the tape drive then wait for it to read.

Let’s assume you have all hard drives and in a setup with absolutely zero redundancy in case a drive fails.

We’re using the Seagate Exos X24 (24TB) drive which is roughly $700 each brand new.

You’ll need 4167 of them to store 100PB. Which puts you at $2,916,900 just for the drives.

Let’s assume you already have the enclosures, racks, and servers for a small datacenter ready to go.

A drive can use 4-9w of power when spinning so assuming all drives are active (to ensure quick data access and data repair) that’ll be roughly 27086w for all the drives at 6.5w per drive. Every month (30 days), that is 19502kWh of electricity used. 40 years is roughly 349,680 hours so that comes out to around 9,471,433kWh used.

Assuming you get some damn good electricity rates at $0.12USD per kWh, it’ll cost $1,136,572 to run just the drives.

So in total, assuming you already have a datacenter with the capacity to install all the drives that runs on absolutely zero power, you’ll spend roughly $4,053,472 over the course of 40 years.