It's an accounting term. You need to report the value of assets of your company each reporting cycle. This allows you to report company profit more accurately since the 2400 drives aren't likely not worth what the company originally paid. It's stated as a tax write-off but people get confused with that term (they think X written off == X less tax paid). It's better to correctly state it as a way to more accurately report profit (which may end up with less company tax paid but obviously not 1:1 since company tax is not 100%).

So anyway you basically pretend you resold the drives today. Here they are assuming in 3 years time no one will pay anything for the drives. Somewhat reasonable to be honest since the setup's bespoke and you'll only get a fraction of the value of 3 year old drives if you resold them.

no, we wanted to be conservative by depreciating somewhat more aggressively than that. we have much closer to 5% yearly disk failure rates.