That's a very good question...
My host's policies are that it does full backups & incrementals thru your first 10GB, only weekly full backups after 10GB, and **NO** backups if you grow past 20GB. Although you don't get charged for the extra usage, you lose features.
We have recently blown past the 10GB point, and I don't like the fact that I've lost my incrementals.
I don't want to purge old attachments, because many of the oldest attachments at this point aren't available elsewhere (old gear manuals, documentation, in some cases legacy software) at least not all in one place like our forum. It'd be a shame to purge 'em.
So... I've been considering using an online backup feature, such as Amazon Glacier, Backblaze or JungleDisk. And not relying on my host at all. Still thinking it thru at the moment.
You know, my PC I'm sitting at right now has ~2.5TB of storage total... Kinda ridiculous to be worrying about 10GB...
