The hard part is not downloading or storing the block chain. It's building, maintaining and guaranteeing the on-disk consistency of the block and transaction index that is necessary for validation.
Well, I suppose you've given it a lot of thought already. But I still have a hard time believing downloading the block chain and creating an index requires all that disk hammering. On my computer, the .bitcoin directory is 1.6GB big. It's big, but not that big. It even fits in the memory of most computers! How can building it require hammering the disk for hours?
You'd believe how much it writes to the disk if you make a new ~8gb disk image, defrag & push all the data to front, and then simply install and run Bitcoin. The blockchain will be fragmented all over the disk from all the writes and erases to the database.