But some vendors are taking steps to give subscribers more control and make migration easier. FreshBooks' McDerment believes it would be easy to move his archived files to another cloud. Rackspace provides a Cloud Files API (application programming interface) that could be used to write a new script, he said. Meanwhile, the API allows FreshBooks to manipulate and arrange its documents just by changing code, he said.

Nirvanix also provides a set of APIs, and it has worked out integration deals with vendors of backup and archiving software such as Atempo so customers can keep using the utilities they're familiar with, said Nirvanix President and CEO Jim Zierick. In addition, Nirvanix offers CloudNAS, a bridging software that can make the Nirvanix cloud look like any NAS (network-attached storage) drive. Using the Nirvanix API set, it mimics commonly used file systems such as CIFS (Common Internet File System) and NFS (Network File System), Zierick said.

Still, migrating a large amount of data from one service provider to another, or from a cloud to in-house storage, is a major undertaking. The basic Nirvanix service uses the public Internet to move files around, but the company has helped individual customers to rent high-bandwidth lines to handle large one-time shifts to the Nirvanix cloud, Zierick said. It has even helped customers load the data on a server at their facility and then physically transport that server to Nirvanix for offloading. The company is now talking to storage consulting firms about offering migration services.

Procedo's Kvidera said demand for migration services, which can cost between US$5,000 and $40,000 per terabyte, is growing rapidly.

But worries about availability and being able to back out of your current platform are actually as old as data storage itself.