Defragmentation is the function of moving data around on a disk to join up parts of files – to remove the fragments. Files are split up by the operating system to fit in the spaces left by deleted files, so there's no wasted storage on a hard disk. While this makes good use of hard disk space, it can slow a PC or laptop down, and make it more susceptible to file errors, because bits of a file have to be fetched from several different places, each time it's needed. Enter O&O Defrag 15 Professional.

Defrag 15 Professional, from Berlin-based O&O Software, has one purpose in life, to put these bits back together, reorganising where they're stored on a drive's surface. When you run it, it automatically looks at all the hard drive partitions on a PC, internal and external, analyses how files are stored and presents recommendations for those drives that could benefit from defragmentation. See also: Does PC speed boost software work?

O&O Defrag 15 Professional's interface is very business-like, smart and well organised, displaying panels with coloured squares to show the different states of sectors on each drive. Buttons are used for all the major functions and there are plenty of secondary pop-up dialogues to help with settings.

On our test Windows PC, the 30GB C: partition (it's small because it's regularly re-imaged for testing different utilities) showed fragmentation of 29 percent, which is pretty high. You should try and run at under five percent.

It's difficult to directly measure the benefits of defragmentation. We set the program to defragment it, which took 26 minutes, consolidated over 45,000 fragments and then showed just 3.6 percent fragmentation. Starting up Windows – which involves a lot of disk access – from this drive before defragmentation took 1 minute 53 seconds and there was no measurable improvement after. See also: What's the best PC optimisation software?

However, this could be because the Windows system files, being the first stored and rarely moved, were not severely fragmented. We also tried running an AV scan before and after defragmentation and the time taken to scan around 450,000 files on the drive reduced from 35 minutes to 30 minutes, a 16.7 percent improvement.

O&O Defrag 15 Professional has its own scheduler, so you can set up regular checks and, if necessary, defragment automatically. If you don't want to leave your PC on overnight, to defragment its drives while it's not being used, O&O Defrag 15 Professional offers screensaver mode.

As the name suggests, this mode runs the defragmenter only when a PC's screensaver is running, so fitting in between active sessions, at meal breaks on when you're away from your machine. The program also stops itself at any time it senses system resources are needed for other tasks, so you shouldn't see a performance hit.

One type of storage which shouldn't be defragmented is a Solid State Drive (SSD). This is because constant writing to the same places in the flash chips it's made from can shorten its life. O&O Defrag 15 Professional recognises this and doesn't defragment SSDs, instead using the ATA TRIM command, which helps with SSD wear levelling, to optimise the drive for best use of its available storage.

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O&O Defrag 15 Professional: Specs

  • Windows XP/Vista/7, 32/64 bit, 256MB RAM, 250MB HD
  • Windows XP/Vista/7, 32/64 bit, 256MB RAM, 250MB HD


£29 may seem expensive for a single-function utility and you can certainly get cheaper defragmenters, even freeware. However, O&O Defrag 15 Professional feels far more thorough than free and low-priced offerings, with extensive logging, feedback and scheduling. The 3-PC Family pack reduces the cost per machine to under £17.