Can't you just write all 0's to the drive or just reformat it? Genuine question here, why would you need to physically replace the drive to ensure security when you can write to the whole thing?
For example, it could binary-patch (either at write time or read time) your kernel image on disk to communicate with the NIC, etc...
... rewrote the hard-drive firmware of infected computers—a
never-before-seen engineering marvel that worked on 12 drive
categories from manufacturers including Western Digital, Maxtor,
Samsung, IBM, Micron, Toshiba, and Seagate.
The malicious firmware created a secret storage vault that survived
military-grade disk wiping and reformatting, making sensitive
data stolen from victims available even after reformatting the
drive and reinstalling the operating system. The firmware also
provided programming interfaces that other code in Equation
Group's sprawling malware library could access. Once a hard drive
was compromised, the infection was impossible to detect or remove.
I have a lot of friends who haven't figured out the whole security-as-a-spectrum thing, and they spend a lot of time giving themselves grey hairs over adversaries that 1) they can't beat, 2) aren't worth beating, and 3) don't care about them anyway.
I expect the "cheapest drive" is not an SSD.
You said "the cheapest drive is not an SSD".
The point is to minimise the money spent on the drive supplied with the machine because you're not going to use that drive, you're going to throw it away.