"The problem is that backing up is very expensive. One needs a tape drive - after all these years there still isn't a better solution for the amount of data one can have an a HD."
Oh come on now!
50 quid will get you at least 120GB of decent ATA drive and another few quid will get you a removable drive bracket so you can take the backup drive out and store it somewhere safe.
Yes, I used to use tape, but hard drives are so cheap these days that it isn't worth the hassle.
Ever tried respooling a despooled tape? The chances of your spare HDD going Tango Uniform are a lot less than that!
And as for using mirrored drives for data safety.......ROTFL!