What you do is you buy a good-sized regular internal desktop hard-drive, and also buy an empty external USB enclosure kit (make sure the kit can handle the drive's capacity, some cheaper external kits are limited to 128 Gb or whatever). For $100 you can get a 160-to-200Gb drive, and an enclosure costs maybe $25. Assembling it yourself is easy and costs quite a bit less than buying an external drive ready to go.
Then once a week: you hook this external drive up, take a couple minutes to store whatever you want on it, and then unplug it from the computer and the wall power. This drive should ONLY be connected to electricity and the computer when you are actually moving files onto it or copying files off.
A hard-drive normally lasts about 5 years before mechanically failing you see--but drives spin much of the time the PC is on, even if they aren't actually in use. This one will only be connected for about ten minutes a week or so--meaning that (in theory) it will last about a thousand times as long.
As for the magnetic coating of the hard-drive platters--they are nearly as durable as magnetic tape is. I have worked with hard-drives from the dawn of the desktop-PC age, and I never saw any of them suffer a spontaneous data-corruption error. They all still functioned perfectly, for both reading and writing as long as they had not already failed mechanically--and these were <100Mb drives that were nearly 15 years old.
...And this really only has to last you about ten years or so. By that time, there will probably be much cheaper+larger storage devices available anyway. Assuming you could get a 200Gb external drive assembled for $150, it would only cost you $15 per year over ten years, not really all that expensive when you consider it. You might even go with using two drives, for keeping two copies of everything.
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