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Microsoft’s (s msft) ambitious Windows Azure cloud is many things — it’s a full-fledged platform as a service (PaaS) for developers. But beneath all that, it is also a huge pool of foundational compute and storage infrastructure for rent.

And that infrastructure layer is fine indeed, according to Andres Rodriquez, CEO of Nasuni, a Natick, Mass. startup that specializes in managing business customers’ storage securely in whatever public cloud best suits their needs. He’s checked out all the public clouds and his conclusion? “Microsoft has a kick-ass cloud.”

That could come in handy now as more tech companies seek to replicate the success of Dropbox, the Silicon Valley darling that has more than 45 million users of its cloud-based storage since it was founded in 2007.

Dropbox provides an easy, inexpensive way for consumers to put their documents, photos and other digital paraphernalia in the cloud and then access it from any device. Dropbox itself puts all that stuff on Amazon’s (s amzn) S3 storage, essentially acting as an on-ramp to a gigantic hard drive in the sky.

Rodriguez says Dropbox is to Amazon storage as a USB stick is to a hard drive: It puts a user-friendly front end onto what is essentially an inscrutable block of storage.

The discussion is relevant again because Google is apparently getting ready to launch its own Dropbox killer. But Rodriguez doesn’t think Google has a shot here. But Microsoft? That’s another story.

“Right now [all these companies] want to be the new Dropbox, but Microsoft is the only company I see as being able to do that,” Rodriquez said in an interview Friday. Microsoft has the front-end smarts to make the on boarding easy. (Outside of Azure, Microsoft already fields SkyDrive consumer storage.) And, unlike many other tech companies, it also has massive amounts of back-end storage in its data centers. It has both the USB stick and the hard drive.

Nasuni is in a pretty good position to judge the relative merits of these public cloud platforms: It gauges the reliability and availability of all the major cloud storage guys and chooses the highest quality cloud storage provider for its customers. It then monitors clouds storage performance, availability and reliability and if it detects problems move customers’ data to another clouds (nondisruptively.) A few months ago the company released a report that put Amazon and Microsoft at the top of a very competitive heap of cloud storage providers, with Amazon edging out Microsoft for the top slot. Ever since then, Microsoft has been all over Nasuni about how to improve its offering, he said.

That’s fairly typical of Microsoft which is, in my opinion, at its best when it’s not in the power position and at its worst when it leads the pack. Many would argue that Microsoft’s strongest efforts are in its mobile phone OS, where it is a decided underdog, and Azure. Where it leads the pack — as in Office and Windows — things can get sloppy. As we saw with Office feature bloat and, well, remember Vista?

Microsoft Azure overall has not gotten as much traction as expected as a PaaS, he said, but the infrastructure is so solid and scalable that Microsoft can profit even in the extremely thin margins afforded by plain-old-but-very-important cloud storage.

Photo courtesy of Flickr user D.Begley