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I would like to know what the damage to a drive can be if a scandisk is not performed before a disk defrag.

I have looked up some sites that say a scandisk should be run to correct any issues that may be apparent on the system, making sure that the drive is free of errors before a disk defrag is done.

I have to perform a defrag on a computer without having physical access to it (using remote connection). I know that the scandisk requires a reboot to the system (causing me to lose my connection) which would be difficult to restart physically if the system does not come back on by itself once rebooted (this has happened before).

Any suggestions?

Thank you.

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migrated from stackoverflow.com Nov 19 '10 at 20:03

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4 Answers

Past implementations of defrag would assume that the basic structure of the disk was self-consistent. If the filesystem was inconsistent, defragging could turn a slightly corrupt disk into an extremely currupt disk - data could easily be lost.

Running scandisk ensures that the disk structure is self-consistent to start with. Though if you check a disk in modern Windows, remember that "completed successfuly" means the program ran and completed, but doesn't mean there were no disk errors. Check in the event logs - for me that's right click "My Computer", select "Manage", then "Event Viewer".

I'm not sure if that first paragraph still applies. Current filesystems seem less prone to these inconsistencies (though they can certainly still happen - especially if you're prone to switching off the power without doing a clean shutdown), and I wouldn't be surprised if current versions of defrag do more consistency checks. Certainly, when I run defrag, I never bother checking the disk first these days.

If you really need to recover data from a potentially corrupt disk, the first thing to do is to get a complete sector-by-sector backup. Anything you try that can modify the disk may go wrong and do more damage. Anything at all that writes to the disk may result in previously recoverable data being lost.

Except, of course, that the best thing is to have a backup before the corruption happens.

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Thanks for the answer. Makes the most sense to backup everything, however we aren't able to do that. We have already backed up essential files. But I appreciated the depthness of the answer. –  Justin Gregoire Nov 19 '10 at 21:14
    
+1 for backup recommendation. By FAR the most important thing you can do. –  Jeff F. Jan 13 '11 at 21:01
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the only reason what springs to my mind right now is that you would want to run scan disk first so that if there are any corrupt clusters the data in them can be recovered and moved and the cluster marked as bad. this way when you defrag the degfrag program will not put good files into bad clusters.

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Thanks for the answer.if a corrupted file is moved to a good cluster or a good cluster recieves a bad file, what would be more damging to the system as a whole. Neither are good I imagine, but what might be easier to treat? and are bad cluster treatable or are you only able to quarantine them preventing fles from being stored there. If thats the case we would need to follow davr 's suggestion and reapir the damaged hardware. –  Justin Gregoire Nov 19 '10 at 21:03
    
a file in a corrupt cluster will be damaged. Scandisk (and similar programs) will have options on what to do with these files (delete, attempt to recover, leave as is etc). i would not imagine that a good cluster with a damaged file would be affected under normal circumstances. basic example: a cluster is like a bucket and a file is like water. a good bucket will hold the water and bad one won't. scandisk will move water in a bad bucket to a different good bucket. if the water is dirty (damaged file) a good bucket still holds it just fine. –  Xantec Nov 19 '10 at 22:24
    
as for what to do with bad clusters, generally there is not much to do. scandisk will mark them as bad so the file system doesn't use them. this generally will not affect performance of the disk because modern disks (at least hard disks) will have "spare" sectors because some are expected to be bad (built in quality control). a sector is the physical hard disk equivalent to the file systems software cluster. –  Xantec Nov 19 '10 at 22:33
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If your machine does not come back up on reboot, then you have other major problems you need to solve before you worry about something relatively unimportant like running defrag. Fix your software or hardware problems first, then run chkdisk (don't think it's called scandisk anymore?), then run defrag.

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Thanks for the answer. We are trying to find solutions to improve performance before we resort to replacement or hardware maintenance. –  Justin Gregoire Nov 19 '10 at 20:41
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With NTFS there is really no need to run a chkdsk since if it is mounted, the FS is pretty much guaranteed to be in good shape. With FAT though, defragmenting it can make a corrupt fs worse.

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This is untrue, NTFS FS corruption is still fairly common on system drives, I deal with it on a daily basis and is by no means "guaranteed to be in good shape". This is especially true with xp and 2003. –  Jeff F. Jan 13 '11 at 20:54
    
Barring some rare and really bizzare hardware problem, the journal makes sure this doesn't happen. A simple crash or power loss won't do it. If you are seeing corrupt filesystems on a daily basis then you need to either fix your badly busted hardware, or more likely, stop writing to the raw disk outside the knowledge of the filesystem. –  psusi Jan 13 '11 at 21:02
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