The first thing you have to decide is how badly someone wants the data on your old tapes. If you think someone may dumpster dive for your tapes and then spend many, many dollars to recover the data with electron microscopes and other cool spy stuff, simple solutions aren't going to cut it.
Let's take a quick look at alternatives good enough for sensitive, but not classified, information for those of us in the real world. I'll rate each approach from 1-5 in hassle, odds of data recovery, environmental impact, and cost.
Running the tape through your drive and telling your backup software to erase, or write nonsensitive data to the tape, can be effective, if very slow. It also won't work with damaged tapes or for tapes you don't have a working drive for. The wear and tear on your tape drives also is an issue.
Once they're overwritten, tapes can be tossed in the trash or even sold for recycling. If the NSA's after you, it may be able to recover some data from previous backups, but it will require specialized hardware, so Akbar and Jeff's Industrial Espionage probably can't. Hassle -- 5; odds of recovery -- 3; environmental impact -- 4; cost -- 3.
Physical destruction is so satisfying, especially the toss of tapes in a bonfire type. Burned tapes are pretty much impossible to read, but the noxious fumes could kill you and land you in the slammer if Ranger Rick comes to see what all the black smoke is about. Hassle -- 3; odds of recovery -- 5; environmental impact -- 5; cost -- 1.
Running the tapes through an industrial shredder or wood chipper also makes them unreadable and the bits can go in the landfill with the rest of the trash. It's emotionally satisfying and can supply your organization with all the confetti you need for years. On the other hand, industrial shredders are expensive. Hassle -- 1 (if you have the shredder or use a drive-up shredder truck); odds of recovery -- 5; environmental impact -- 2; cost -- 3.
Degaussing, or bulk erasing, wipes the magnetic domains on your tapes by exposing the tapes to a strong, alternating, magnetic field. Back in the old days, when we made cassette recordings of LPs and FM broadcasts, running the cassette through a $30 Radio Shack bulk eraser cut the background noise substantially, especially if you were reusing a tape.
I recently had a client pull the bulk eraser they previously used on floppy disks and 9-track tapes out of a closet to use on some SDLT tapes they were retiring as part of a switch over to LTO (Linear Tape-Open). I convinced them to try to read a tape after it had been through the bulk erase cycle and it worked like a champ. Today's tapes have 2 to 3 times the coercivity of older media and require a much stronger magnetic field to erase.
You can get degaussers suitable for today's tapes, and even hard drives, from vendors like Garner Products for $2,000 and up. Also bear in mind that, while you can degauss DLT (Digital Linear Tape) and SDLT (Super DLT) tapes and reuse them, LTO tapes have a prerecorded magnetic servo to align the heads to tape tracks. Degauss these tapes and they're useless. Hassle -- 2; odds of recovery -- 5 (if you have the right degausser); environmental impact -- 2; cost 4.
If the NSA is after you, I suggest degaussing AND shredding.