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Linux and large hard drives

I just finished rebuilding the backup server, and I discovered something new about Linux which I didn’t know (although I did assume it was possible).

I’d tried googling for the answer to this question without avail, so I’ve blogged it here to hopefully answer the question for someone else in my same situation.

The problem is you have a large hard disk, let’s say 80GB. The computer’s BIOS is old and can’t detect it properly. The question is, can Linux access / mount / detect a hard disk larger than the BIOS supports and can detect?

Well the answer it would seem is proved to be yes, in at least some circumstances.

I’m not saying this will work for everyone, because I’m not a Linux expert. But it did work for me, and I’ll give my educated guess at why.

When Linux boots (well actually just before it boots) the computer moves from 16bit to 32bit mode (“protected mode”, a 386 feature which stuck and exists even in the latest x86 AMD and Intel processors). When this happens, the BIOS is rendered fairly uesless (as I recall, it was a long time ago I was interested in writing my own 32bit OS!) and so the Operating System starts accessing hardware through “IO” ports, special registers (again, as I recall) in the processor. Through these, it can access all the hardware without going near the BIOS, so even if your HD is not supported by the BIOS, it may well be accessible by Linux, or any other 32bit Operating System.

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