And right you are.

But what do you need:
1) a backup system
or
2) a data storage system?

1) Backup system
Implementing a backup system requires several layers of security, for example:
- RAID 1 (mirroring) for security against drive failures, if possible with a hot spare, if possible hot-swappable.
- Change logging for security against lost changes between backups (aka incremental backups, but transaction logging is also in this category). These logs should be stored on a immediately accessible (though not neccessarily very fast) medium, if possible on both a local and remote system.
- backing up the complete data set on a regular basis, if possible on a multi-generation tape set, if possible in a long, medium and slow cycle.

If you need to backup consider a tape backup system. for example ADR offers a tape unit (IDE) for €800, the tapes cost €109 per 120GB. Tapes are the cheapest way to secure your data, mainly because they are easily reusable.


2. Data storage.
You need data storage if your system generates data which isn't going to be used regularly, which don't need to be on-line, but which may be needed in the future. An archive thus.

Keeping data for an extremely long time (>10yrs) is quite difficult. Not only the media needs to last all the time, but also the devices for them. If the drives break down they need to be repaired and/or replaced (do you still have punchcard reader lying around?). Last but not least: the data is in some format (ZIP, JPG, MDB, DOC, TXT), it is quite likely that some of these formats will not be supporterd in the far future.
There is a lot more than it to data storage, but i've tried to point out the main issues.

About the media:
CD's are prone to scratching, and to CD rot (detoriation of the reflective layer and/or the dye). The same goes for DVDs. As far as I know CD's kan be kept almost indefinately if kept under good circumstances (dry, dark and cool), but I'm not sure about CDRoms. If you make a double CD-set and store one set on a remote secure location I think you're quite safe.

HDD's are very fragile. The magnetic information will also deteriorate in time, but will last perhaps a decade.

Tapes break down, get dirty, fold, wear etc etc. The information on a tape will be readable only for a couple of years. Tapes are useful only if you use them to create backups of a large quantity of data regularly, to recreate a data set as up-to-date as possible.

IMHO the best way to store data for a extremely long period of time (>10Yrs) is on CD's, followed by DVD's. Then comes CDRoms. DVDRom is still out as there are still too many standards and the technology is still pretty young. Any magnetic medium is out as magnetic information deteriorates with time, and deteriorates every time the media is accessed.