hard drives that are in constant use, have their data moved around a lot stay healthy longer, because of re-magnetizing the data. That's why a harddrive light is never really off on a Windows vista/7 computer.
Harddrives of the nineties, below 8GB used to have a data retention life of 4 to 6 years. Harddrives today are 2 to 3 years tops, before they start corrupting or losing data.
Maxtors are the worst.
SD cards keep data for a long time, but get corrupted when a lot of data is written over sectors. You may know SD cards are flash memory cards with MLC's (meaning there's multiple of data bits stored in one cell, leading to degradation when a lot of read writes are being performed).
CD-R's have a shelf life of about 3-5 years, 7 years tops. DVD's of 5-7 years, 9 tops.
But, it's still better than a floppy disk!