I have an HDD with… I think 4 TB laying around. What would be the best option? To just plug it into the server and leave it there?

Thank you, I’ve downloaded the .md for my Obsidian notes :-) Great starting point!

Same

Why did you measure inside the cabinet? Don’t you want to dampen the perceived noise outside of the cabinet?

Physically speaking, 3 cm will absorb all frequencies above 12000 Hz, and dampen everything beneath at least a bit, ignoring the absorption capatbilities of the material. I’m no expert in the field of material sciences, I just know by word and experience (insert “trust me bro” here) that its absortion capabilities are really good.

It will lessen the perceived sound by a measure, but do not believe, that all sound will be gone. But a 3 cm-thick paddening all around will to a good job.

Think twice about glueing. Glue is nasty to remove, maybe just push cut-to-size pieces inside of the ducts to make them fit? You could make ones, which you make snug around the cables leaving the cabinet?

Regarding sound proofing:

Measure the loudness level in dB (A) [the A is important, don’t get measurements in just “dB”], and if possible check the frequency spectrum. RoomEQ Wizard is a free software, that does a great job. Otherwise, an app with similar possibilites does the job aswell, and you don’t need to hassle with an external microphone.

This is important, so you can check which frequencies produce the most noise, thus you can proof more effectively. Example given, you’ve got a noise level of 120 dB (A) around 100 Hz, you would work on this task differently, as if you’ve got 70 dB (A) around 1000 Hz.

The higher the frequency, the shorter are the wavelengths, which leads to thinner dampening methods needed. The lower the frequencies are, the bigger the more absorbant the material should be.

Regarding non-flammable materials with good absorbation, check out Basotect, which you can buy and cut to your needs.