I was just about to say that CrashPlan is restricted to the internal drive of a computer & is therefore limited to the maximum size of hard drives (e.g. 1-2 Terabytes for laptops), but apparently CrashPlan lets you backup external drives now too:
Having used CrashPlan's personal offerings for many years, I recall that they have always allowed the backup of attached storage either from hard drives or network mounts.