Weirder than that, there is a second area on the disk which
lets you do the same thing<p><a href="https://en.wikipedia.org/wiki/Device_configuration_overlay" rel="nofollow">https://en.wikipedia.org/wiki/Device_configuration_overlay</a><p>So there are two hiding places instead of just one.
If you're curious, I think this is the original proposal from ~ 1996. It's amazing to see all the old hardware vendors on these things. Gateway and Maxtor are dead now (Maxtor is on the ATA-3 specification). I like that this covers two pretty common use cases. I wonder if the current "sleep" technology of computers does this with the RAM, or if they use another more modern mechanism.<p><a href="http://t13.org/Documents/UploadedDocuments/technical/d96137r0.pdf" rel="nofollow">http://t13.org/Documents/UploadedDocuments/technical/d96137r...</a>
> <i>HPA can be used by various booting and diagnostic utilities, normally in conjunction with the BIOS. An example of this implementation is the Phoenix FirstBIOS, which uses Boot Engineering Extension Record (BEER) and Protected Area Run Time Interface Extension Services (PARTIES).</i>
If you're wondering if NSA or others are using it for their purposes, yes, they are:<p><a href="https://www.schneier.com/blog/archives/2014/02/swap_nsa_exploi.html" rel="nofollow">https://www.schneier.com/blog/archives/2014/02/swap_nsa_expl...</a><p>Warning: contains TS classification markers. If you have clearance you might not want to click that.
HPA was also a good way to overprovision SSDs for better write performance before TRIM existed (although today's SSDs are so fast I would no longer worry about overprovisioning).