Disadvantages to CD-r and DVD-r are that you can scratch the actual data surface off easily.
Disadvantages to internal hard disk are, theft of computer, electrical surge can take out both, OS glitch, virus, etc can take out both.
Disadvantages to external drive, its still vulnerable to virus and os problems, easier to drop and crash when you move it around.
Disadvantages to USB flash drives or other Flash media are space and speed and dare I say EMP <img src="/images/graemlins/smile.gif" alt="" />
Disadvantages to offline are loss due to the inline provider not backing up, same for the friend of ISP.
Good reasons to always use multiple (as was stated)
A couple others are tape and (very unreliable) zip drives.