[trotsky]

any content that is DRM'd is also unique.

In most cases this isn't true. The computation involved in keying media on the fly while it's being downloaded is not insignificant when considered in volume. The added pain of storing everyone's unique keys also discourages this behavior. At worst you'll see different keys being used by region or datacenter, or perhaps key rotation on a weeks-months scale.

Some media (both DRM and non-DRM) will be trivially unique because of metadata like purchaser info or music tags. In some cases this makes the first block unique but all later blocks are deduped, in other cases you need to be somewhat content aware so you can treat the header data separate from the real media data. This also allows you to catch a lot of data people ripped themselves using standard settings.

You can save on operating system and application files, but it isn't 60%.

While I agree that photos and videos will be the bulk of their problem, I don't think that ruins their premise. The question is if their userbase will be significantly overweight on heavy media creators. If it's a standard distribution, I wouldn't be surprised if a majority of people were under 10gb unique and 70%+ deduped.

[nikcub]

I tested it on iTunes. bought the same television episode on two different accounts on two different laptops and compared them to find that they had 4-5 bytes in common.

Not sure how it works in WMP. It would be common in blu-ray rips, though.