The USB hard disk was a great invention when it came to adding capacity to a local PC, or doing backup, or moving data around. But it was only a data-store, so it had its limitations.

In particular, it was hard to install applications there - because most modern apps spread their junk all over the system drive. Plus, a hard disk needed mains power, all of which meant it didn't work well as a portable personal workspace.

So for those of us who need to work at different locations but don't want to - or can't - do everything on the web, there hasn't been much alternative to carrying a laptop. Indeed, it seems that many laptops rarely if ever get used on batteries - they are simply a way of moving one's workspace from office to office, or from office to home.

All that is changing, thanks to technologies such as software virtualisation and the development of small form factor (SFF) drives that need less power. It means you really can take a hard disk and turn it into a 'portable desktop environment' - and that's what Seagate has done with the FreeAgent Go.

What you get is a pocket-sized enclosure, holding a 160GB 2.5-inch hard disk, spinning at 5400rpm and connecting over USB 2.0. It can transfer data at over 10 megabytes per second (MBps), which is several times faster than a USB flash drive.

It does require a fair bit of power, and not all USB 2.0 sockets will be able to supply enough. Seagate therefore supplies a cable with two USB plugs - if the main one is not enough (one was sufficient on our desktop PC, but the laptop needed two), you can plug the secondary into a spare port.

Once powered up, one end of the drive glows orange - if it is flashing, it's not getting enough power. Then, if your PC supports autorun for this type of device - and as long as it's running Windows 2000 or newer - up pops a menu with the first option being to start its Ceedo interface. If you don't have autorun enabled, you can go in manually and start the software.

This is where it starts to get clever. The Ceedo software, which Seagate has licensed from its eponymous developer, creates a virtual area within the system - you can install and run apps, and sync data onto it, all without any permanent effect on the host PC.

Then when you dismount and unplug FreeAgent, it cleans up after itself and is ready to run on a different PC, with all your apps and data just as they were.

You can also install a small agent on the PC which automatically starts Ceedo, password-protect the drive, or install Seagate's FreeAgent tools - these let you define which directories you want synced onto the drive, encrypt files (using 256-bit AES), run diagnostics and so on.

Applications are installed from Ceedo's website and are specially packaged to install and run on a removable drive. Several categories are available, covering most needs, such as web browsers, FTP clients, instant messengers and email, and FreeAgent gets its own quick-launch bar on the screen, listing what you have installed.

Of course, most commercial software vendors still expect you to install their app on a specific machine. Ceedo gets around that by concentrating on open source and shareware, with its less restrictive conditions - although it also supports some widely installed apps such as Internet Explorer and Outlook Express, which it runs from the host PC but using data stored on the FreeAgent.

There is a range of apps available, but they weren't always the latest versions, and running that supplier's standard update process did not always work on the FreeAgent. For example, we were unable to update the Opera web browser from version 9.1, which Ceedo supplied, to the more current 9.24 release.

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