My images reside on an external hard drive, from where they are accessed and manipulated by Lightroom. Every night the top-level Photo folder is backed up to another external hard drive - just in case. This backup drive is never accessed unless something goes wrong which (so far, knock on wood, etc.) it hasn't.

The backup drive is filling up, and I'm wondering when it makes sense to replace it with a larger one? I have a recollection that in the days when a gigabyte was a BIG disk (my original IBM PC has a 20 MEGAbyte hard disk, which was an expensive upgrade from the standard 10 Mb disk) there was a rule of thumb that you shouldn't let your hard drive get much more than 90% full. But in today's muti-terabyte drives, 10% of, say, a 2TB drive is still 200 GIGAbytes of space. Even with the largest Nikon or Canon sensors, 200GB will still hold a LOT of images.

I understand that with an "active" disk that gets written to and read from a lot, with constant changes to files, there needs to be disk space for the operating system to do its work properly. But for a backup disk, where basically each file gets written once and then is left untouched, unless it gets read off to replace one that got corrupted or lost, how much "spare" room does the disk really need to have for these single-write and occasional single-read operations to work properly? (I assume the same factors would apply to my external movie hard drive, to which I rip my DVDs so they can be viewed at will on various monitors around the house using Apple TVs - movies get ripped and written once, and then just read on demand.)

Thanks for any thoughts.