[jmathai]

The way we dealt with that was for storage systems like Dropbox we store originals there but cache all of the thumbnails in an S3 bucket. It also didn't make sense to store thumbs in a user's Dropbox account.

All in all we treated every storage system independently but it was easy for us because that was the design from the beginning.

Even in our case the vast majority of consumers use storage we provided. We even support migration between storage services and that got some good use but nothing of any significant scale.

Drop me a note when you're in the Bay Area. @jmathai on Twitter or contact info in profile.