If it's only 20 pages, I'd actually guess not. HDD + controller, for a minimum of (say) a 20 gig drive (I couldn't find any smaller ones), I'd guess would be a lot more than the extra hundred megs (tops) in the RAM that it already needs to have anyway. So it only makes sense to have a drive if you want to store a LOT more images than 20.

Maybe it needs a drive anyway for some other reason (like the software that runs it lives there instead of on a CF card or something), so it ends up being cheaper and easier that way?

100 megs? A single letter-size sheet is 30 megs at 600 ppi.

Okay, so 600 megs. An extra gig of RAM has to be cheaper than a hard drive + controller you wouldn't otherwise need, right?

Many people occasionally copy/print multi-hundred page documents. That runs to many gigs of RAM, $30 just for the DRAM chips. And then you need to design what amounts to a custom PC motherboard to talk to them. The price could easily run to $75, not counting the enormous engineering costs.

Or you buy a $40 ATA drive and connect it to your embedded processor's built-in interface.