Some people do it, and unlike many hosts, DH actually allows it.
Some people probably prefer to keep their data more private and feel better about keeping it at home. Well, at least until their home catches on fire or something.
External USB hard drives are cheap and the data moves a lot quicker to and from it. Your upload & download speeds could be a deciding factor in whether or not you want to do it.
If you're talking about smaller amounts, even media like SD cards come in handy. I keep a 512 MB SD card in my laptop, which I regularly update with important files that change frequently. (That's also in addition to backing everything up on the external drive.)
You need a file while DH or your ISP is down, or right after your modem/router/etc... fails.
I guess I'm just used to having fast access to my external hard drive, but I can certainly understand the desire to do it. Especially for people like me that are using less than 1% of our disk space.
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