There appears to be some confusion in this thread. Since long-term archiving of digital data is essential for me (and no doubt for many of you), I have investigated this in some detail:
Hard drives. Great for
short-term storage, backups, but not for long-term archives. Biggest problem is not de-magnetisation of the disks (half-life is typically ~70 yr) but aging of lubricants etc, resulting in mechanical failure after 3-5 yr. The problem is actually exacerbated by storing the drives powered off, off-line. Best strategy is to store multiple copies and re-write the data every 2 yr or so. On-line discs are ok for backup, but too volatile for archive.
SSD/flash drives lose their charge over time and typically last 5-10 yr unless re-written.
Cloud storage requires high bandwidth connection, useful for smaller amounts of shorter-term backup, but not for archives. Do you trust the cloud storage provider to still keep your data safe 20 yr from now? Will the company still be around?
Optical media - considered to be the ultimate digital archival solution, adopted by libraries etc. Note that there is a huge difference between pressed media and write-able discs. Write-able CDs and DVDs tend to use organic dye that deteriorate in time - could be as bad as in a few years only. There are some exceptions,
m-disc being a notable one which uses inorganic dye (in the form of "rock powder"!), but requires specially designed writers with extra powerful lasers. Regular blue ray discs also use inorganic dyes, so should be fine. There is also an m-disc BD version, but it is not clear if there is any longevity advantage yet (although they are marketed as 1000-yr solutions). Stay clear from the
BD LTH discs, however, which are developed to use the same manufacturing processes as CDs/DVDs with organic dye, to significantly reduce the manufacturing costs. They have similar lifetimes as the regular write-able CDs/DVDs. Note that pressed media are different and should have lifetimes on the order of 100's of years. They are typically not useful for archival purposes, though, since you need a very big series before it becomes economically feasible to press discs.
Magnetic tape - the most economical archival solution for huge data sets is to use magnetic tapes, such as
LTO. There are archival WORM tapes ("write once, read many") that are certified for 40 yr storage, and you can get a 2.5 TB tape for less than $60. These tapes are typically used by huge data centers. A problem is of course that the tape drives are typically > $1500. Perhaps they are rentable.
A good practice to protect from bit errors is to save redundancy information together with the data, in the form of error correcting codes (
ECC). Popular software to generate these are
ICE ECC,
MultiPar, and
dvdisaster (the last optimised for ECC on optical media).
Some
general, disorganised rants about archiving that I nevertheless found useful.
In summary, hard drives are good for short term backups, (non-LTH) BDs for long-term storage of limited data generation rates (less than ~ TB/yr), magnetic tapes for large data rates (several TB/yr).
