The other problem with spinning hard disks is that, relative to memory chips, they are extremely slow.

A downside of flash memory like a USB stick is that it reads fast, it is relatively slow to write to, and has a finite life in terms of writes. For normal use, this isn't an issue but if very heavily written to by an application or operating system it will eventually fail. I've heard the figure of about 100,000 write cycles being a reasonable life expectancy for a flash drive. I'm sure Wikipedia could shed some light.