Unfortunately this isn't adequate. Drives contain "extra" storage space above rated capacity, as well as reduce delivered capacity, which gets periodically unm

Unfortunately this isn't adequate. Drives contain "extra" storage space above rated capacity, as well as reduce delivered capacity, which gets periodically unmapped by wear leveling, or due to errors, etc. This is a bigger deal on SSD than on spinning-rust, but even with rust, it was an issue (esp with 4K sectors, or when worried about things like keys). It's certainly better than nothing, but not sufficient practice in a business or high-security-professional environment. An attacker is willing to load custom firmware onto the drive, or to move the chips into a new device, or otherwise read it out raw, and will have access to more material than your dd can write. The standard should be "can I prove this will work reliably, given all the layers beneath me", and for that, the only adequate answer is physical destruction. You could possibly design a drive where you're guaranteed to know if everything is fully deleted, and as long as you trusted the design/implementation/current-status, you could rely on it, but then you'd have a $100k 100GB SSD. So much easier just to replace old drives, or to guarantee that nothing interesting every touched the disk unencrypted, or ideally both.