Many moons ago a fellow I worked with who had been stationed at an airbase told a story about how they decommissioned their hard drives. He said that they took them to a double fenced area with armed guards, put them and a number of thermite (spell checker wanted to call it termite) charges into a 6 foot deep hole that had been dug with a backhoe & melted them into a heap of slag which was them buried.
Hammers are so last century.
Plus thermite, you got it correct https://en.wikipedia.org/wiki/Thermite
As for the time that I served in a comm center (STRATCOM Tehran, & USAEPG Ft. Huachuca) the thermite plates were controlled by serial number and tagged. There was a shelf life and had to be replaced. We used the old ones to ignite over stuff we normally just burned. True, the ingredients can be purchased online, but the actual ready devices used primarily to destroy classified equipment (crypto) in emergency situations is controlled and must be accounted for in each day’s audit before a shift change. Just as each page of any classified manual must be audited and page counted manually after it has been checked out of the safe before returning to it.
Just this one point about how the H did Bradley Manning get anything out of a comm center, especially one in a combat theatre is beyond me other than the fix was in and he was the pastie, as we are finding out in more ones that one.