I'm surprised no-one has come up with a product for this yet.
Imagine your favourite 3.5" desktop hard disk. It's about 100mm wide, about 145mm long and about 26mm high. Normally the length isn't too much of a drama, in your average desktop PC (ignore the use of 3.5" drives in servers and hot-swap bays for the moment).
Now imagine your normal laptop drive. It's smaller in every dimension (just 70mm wide, 100mm long and 10mm tall. At those sorts of sizes you could easily fit two laptop drives inside the same space used by the 3.5" drive.
You'd even have about 40mm of free space at the "back" of the drive (or 10mm under it!) for some circuitry
All this is building up to a concept. It's a 2 drive mirror set (RAID 1) using a pair of laptop drives (which are up to 250GB nowadays) and a small onboard controller that does RAID 1 (and probably ONLY RAID1) in hardware. Maybe there's a jumper on it to make it a RAID 0 set for speed instead, but for my thoughts it's RAID 1 only. On the back of this funky drive is a single SATA connector for data and a SATA power connector (laptop drives use lots less power than desktop, so a single connector will provide more than enough power).
Why?
Because I'm sick of disk failures. I want the PC I build for my mother to not need RAID drivers, multiple cables, multiple disks etc. But I do want to be able to recover from a drive failure without manual rebuilds, restoring backups or user pain.
It should use less power, overall, than a desktop drive (less heat). It should have great read performance, but the write performance will suffer a bit (64MB of cache on the controller would help here though). And when a disk fails, you don't lose your data.
Hmm.
How about putting 6 laptop drives in a DVD drive bay? A DVD drive is 145mm wide (just more than twice the laptop disk), and 40mm high (easily space for 3 layers of disks, with room for boards, eject levers etc). Make it do either a 6 disk RAID 10 set (huge performance gains) or a 5 disk RAID 5 set (which theoretically will be very efficient). That's a 750GB RAID 10 drive, so lose a disk and it still works FAST; or a 1 TB RAID 5 disk with a slot for a spare, and way more performance than a single 7200rpm desktop drive.
I'd pay the premium ...