When you do backups, you have lots of choices for storage media. What to choose involves a lot of tradeoffs: Cost, durability, access speed, convenience of use, physical size, and storage capacity are some of the factors to balance.

In many ways, the good old magnetic hard disk drive is the best. It has high capacity and good cross-system portability, and it isn’t easily damaged. The cost isn’t outrageous, though it’s not the cheapest option. A downside is that it comes with an interface, and in a few years it might be difficult to plug today’s drive into a new computer. USB has been king of the hill for a while but could soon lose its dominance, and we don’t know yet what will replace it. A drive can fail catastrophically if the read-write head touches the disk, especially with rough handling during transportation.

The USB memory stick or flash drive is small and cheap. It has its place in a backup scheme, since it let you make a quick copy of a few gigabytes and take it to a safe place, but it’s poor on longevity. It’s based on EPROM technology, which is prone to electrostatic discharge over time. Don’t count on it for long-term storage. The more times you rewrite it, the more likely it is to fail.

What about larger solid-state drives (SSD’s)? These are generally better made than memory sticks and have a better chance of survival. These come in two kinds. The kind based on dynamic RAM requires that its batteries not run out, or it will lose everything. These shouldn’t be considered for archival storage. The kind based on flash or EPROM technology is steadily improving, but I don’t think it’s proven itself for archival use yet. The analyses I’ve seen focus on ways the drives are kept from being exhausted by too many writes but don’t say anything about life on the shelf and the attendant electrical discharge issues. They may become a serious form of archival storage in the future.

CD-ROM has some nice features; it’s a well-standardized format, it’s immune to magnetic fields, and it’s separate from the hardware that reads it. The only problem is that its capacity is tiny by modern standards. You can use CD-R (recordable once only) or CD-RW (rewritable) drives. For long-term storage, CD-R is significantly more reliable. “Archival grade” discs from a reputable manufacturer are best; these use more stable dyes and, with top-quality discs, a gold reflective layer rather than silver or aluminum. Avoid the cheap makes if you’re storing anything important. The greatest risk is that the disc will not be written properly, so always verify after copying.

With DVD-ROM, you get the same advantages and more storage for a higher price per disc. The higher bit density means somewhat more vulnerability to damage, and DVD’s are more encumbered with trade secrets and protection schemes, so there’s a slightly greater chance of obsolescence. Fortunately, the more annoying features of DVD’s, such as region codes and secret specifications, don’t apply to the data discs. There are more varieties of writable DVD’s including DVD-R, DVD+R, DVD-RW, and DVD+RW. The “plus” formats are generally considered worth their slightly higher cost. The same considerations as with CD’s generally apply. Avoid double-sided discs; they’re more vulnerable to damage.

Writable Blu-Ray discs provide even more storage, and they have some advantages over DVD for durability. They’re still expensive, though, and few computers have drives that will write them. Blu-Ray uses the full thickness of the disc, unlike DVD, which uses only half in a single-sided disc. (There are no double-sided Blu-Ray discs.) There is a two-nanometer hardcoat over the reading side, which provides extra protection against scratches and marks. With a triple-layer BDXL disc, you can get 100 gigabytes on a single disc. A few drives are now available that can do this, though I’m seeing prices of over $100 per disc.

Magnetic tape can get you really large amounts of storage. The drive costs some money, but tape is cheap and will last for many years if properly stored. The drawbacks are that access is slow, tape is breakable, and there’s little compatibility among different drive models. Tape backup makes sense mostly for fairly large installations, where physical storage space is an issue or multiple hard drives would be needed to back everything up.

With any medium, longevity estimates don’t mean that you’ll get the specified number of years and then they’ll give out. It’s better to think of them as a “half-life.” Some devices will die early, others will far outlast expectations. I have a number of audio CD’s from the eighties that play perfectly. Others haven’t had such luck with much newer ones. How well you store your media has a huge effect on how long they’ll last.

Suggestions: Flash drives are great for keeping extra copies in safe places, but don’t rely on them for long-term storage. Writing your most important files to CD or DVD as an extra backup is a good plan. For your main backup, a good-quality magnetic hard drive is your best bet. For an IT shop of moderate or bigger size, having tape as a second-tier backup makes it practical to keep older backups around.