By Mark Ward
Technology Correspondent, BBC News website
Zoe Kleinman and Mark Ward find out the safest way to delete all data from a hard drive
If you want to sell your old computer there is one over-riding problem you have to solve - deleting the data on its hard drive.
Sitting on that drive will be the sum total of your, and your family's, digital life. It will contain login names, passwords, sensitive documents, cherished pictures and every other sort of information.
Any and all of that could prove valuable to hi-tech criminals who use that personal data to steal from you directly or tarnish your good name by using it to sign up for credit cards and the like.
The BBC's technology team and Working Lunch tried to find out the best way to delete that data or make it so hard to get at that the criminals give up in favour of an easier target.
We tried three different methods of removing the data: using the delete key, reformatting the drive and over-writing the data with zeros with a free utility program.
The test subject was an 80GB Hitachi Deskstar ATA/IDE drive that has been the main drive for a PC since 2004.
The PC has been used to play games, browse the web, store pictures, music and so on. Windows XP Pro has been re-installed on it a few times and prior to the test it had 28% free space left and the data was heavily fragmented.
With the help of data recovery firm Kroll OnTrack the data on the drive was copied on to three identical drives. Each one was subject to a different deleting method.
Over-writing the data on a hard drive can take a long time
The first trial involved using the delete key to get rid of the data. This proved surprisingly difficult.
There were a lot of elements, such as the Documents and Settings folder, that Windows will not let a user delete. Even one with administrator privileges. Others, such as IE, Outlook and MS Office also resisted deletion.
Some were too big for the recycle bin and had to be deleted permanently straight away.
After about an hour, with the aid of Windows' Add or Remove Programs utility, we managed to delete so much that the machine crashed and would not re-start in any usable fashion.
Reformatting the second hard drive was much more straightforward. We used a Windows XP Pro boot disk to fool the machine into thinking the operating system was being re-installed. During this process it re-formats the drive in preparation for the fresh install. Time taken was about 40 minutes.
For the third drive we turned to a free utility called Darik's Boot and Nuke. This overwrites every sector on the disk with zeros in a way demanded by many government agencies. Darik's utility is free but there are many other commercial versions that do the job.
Setting up and running Darik's is not for the faint-hearted or technically unsophisticated. It also takes a long time. Writing zeros on the 80GB drive took almost two hours. Bigger drives will take longer.
The drives were handed over to Kroll OnTrack to see how successful the data removal turned out to be.
For two out of three the answer was: not very.
Deleting data is difficult so your digital snaps are pretty safe
About 20GB of data was removed using the delete key and almost all of this was recovered using "simple" tools, said Rob Winter, chief technology officer at Kroll OnTrack.
Of the data 60GB of data on the drive that was reformatted, Kroll was able to easily get 40GB of it back. Mr Winter was confident that the rest would be recoverable with a little more work.
By contrast, none of the data on the over-written drive was recoverable.
"Unless you wipe the drive with specialised software you are not going to remove the data on it," said Mr Winter. He added the warning that the software only works if it is used well and the drive is working normally. User error or problems with the drive could mean that some data is left intact.
Windows' method of treating data is behind its stubborn resistance to deletion, said Chris Boyd, head of forensics at security firm Detica. He said pressing the delete key did not mean that the file was removed.
"It just marks the file as deleted, all the data is still there," he said.
Pressing delete just means that the space can be over-written if a PC needs a place to put some fresh information. On a big drive it could be weeks before that space is needed and overwritten.
Mr Boyd added said that when a file, such as a Word document, is being worked on Windows saves temporary versions in many different places.
As a result, he said, deleting one version of a file may leave those earlier versions intact. Recovering that information is very straightforward.
The end result seems to be clear. Writing zeros to a file removes the data, provided no mistakes are made. For those that want to avoid all trouble when selling a PC, replace the hard drive with a blank one.
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