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Is it safe to de-fragment your disk when you have low disk space? What could be the consequences? What are the advantages and disadvantages of de-fragmenting. Is it possible to resume de-fragmentation of a disk after a few days? Can de-fragmentation of large disks be simplified as it takes lot of time to complete? Is it possible to lose data after de-fragmentation? Will it reclaim my Disk-space?

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" Is it Safe to De-fragment your Disk when you have low disk space, what could be the consequences?"

The defragmenter will not complete the defrag efficiently if you have low free space if you use the Windows defragger. But you can run a third party automatic defragger which defrags under less than 5% free space and get the job done without any problems.

"What are the Advantages and Disadvantages of De-fragmenting.Is it Possible to resume de-fragmentation of a disk after a few days. can De-fragmentation of large disks be simplified as it takes lot of time to complete. Is it possible to lose data after de-fragmentation. Will it reclaim my Disk-space."

There are no disadvantages, advantages are that the performance will be smooth and data access fast. With intelligent automatic defrag technology it's also possible to have a program defrag in the background without being resource intensive and will also defrag very fast as compared to the built-in program. Defrag doesn't create any new diskspace and there is no risk of data loss. Just ensure that you have a power backup for the PC.

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The most annoying consequence will be that defragmenting (since there will be files moved around) will be very slow and take a horrable amount of time. Advantages are, that your files are not fragmented on the harddisc and this will lead to a speedup unfortunatly with very low disk space the fragmentation effects increase dramaticly so that your hard disk will get fragmented again very fast and all defragmenting will be for nothing. There are some commercial programs who promise better results, but i can't tell you if they are any good. And to the last question: it will not recraim your disk space. The amount of free space will stay the same.

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Actually, because the hard disk is divided into 4K blocks it is possible to reclaim some space, because each fragment in a file can waste a small amount of space at the end of the fragment (up to 4K-1). But it's unlikely to be anything measurable. –  Joel Coehoorn Aug 31 '10 at 17:01


It depends on the drive. Defragmenting with low space will cause it to take exponentially longer to finish because there is less room to move files around. This is usually not harmful, only inconvenient because of the performance hit, but it could be harmful if the drive uses flash-memory like a flash-drive, memory card, or SSD due to their limited number of write cycles, which would cause it wear out faster than if you avoided defragmenting.

What are the advantages and disadvantages of de-fragmenting.

If the drive is a traditional hard-drive with spinning platters and moving heads, then keeping the files contiguous (all in a row) will help avoid seek times which can improve performance whereas having the file spread out all over the disk would require more time to read all the pieces. If the drive is a solid-state drive like a flash-drive, memory card, or SSD, then there is no performance advantage.

An advantage that all kinds of drives will have from defragmentation is an increase in data-recovery success. If a file is fragmented, then recovering a lost/deleted/etc. file is highly unlikely, but if the file is contiguous, then it becomes much more likely to succeed.

What you choose to do will depend on what kind of drive it is and what is stored on it (i.e., how important are the files, will you need to do data-recovery, how frequently do the files change and/or are written, etc.)


That depends on what you mean. You can defragment at any time, but as files are modified, the program needs to constantly update its disk-map to know what the disk looks like. Back in Windows 98, this was often manifest by the defrag program constantly starting over from the beginning every few seconds because something wrote to the drive, so users would have to try their best to quit every program possible before running it. These days, it is not made obvious, but the same thing is basically occurring behind the scenes, but defrag programs are a little more intelligent and can dynamically adapt to disk changes without having to start over from scratch (though you will often have to refresh/reread/re-analyse the disk to see the current status).

It is usually easiest/safest to just stop the defrag operation and start a new one later on, even if the system is in standby during the break.


The more free space you have, the faster and easier the defrag will be regardless of the size. Some programs may have different types of defrag options that let you reduce the amount of work, but in general, the larger the disk, the longer it takes. Of course keeping the drive defragmented regularly will reduce the amount of work that needs to be done to just the files that have become fragmented since the last defrag.

Is it possible to lose data after de-fragmentation?

Of course; it is always possible to lose files in general. Not surprisingly, because defragmentation moves files around, it has a higher risk of losing files than in general, however most (if not all) defrag programs (at least any reputable one) will copy the file to the new location before deleting the original. The better ones implement a sort of ACID system to make the moves transactional (i.e., they make sure that the new files is completely written and linked before removing the old on).

To minimize chances of problems, there are a few simple things that you can do/avoid when defragmenting:

  • Never do it during storms or while on low-battery (i.e., a chance of the system shutting down abnormally during defrag).
  • Quit as many programs as possible before defragging to reduce locked files, disk writes, and system crashes.
  • Maximize free space.
  • Perform the defrag regularly to minimize how much needs to be defragged.

Will it reclaim my Disk-space?

Huh? No, not generally. Defragmenting only makes sure that files are stored contiguously (i.e., all in a single row) as opposed to being spread out all over the disk.

In addition to making the files contiguous, it can also make the free space contiguous which while not creating new free space, makes better free space because new files can be written contiguously instead of having to be fragmented.

That said, it is possible to gain new free space under very specific circumstances. If you are using an NTFS volume and the MFT happens to bloat up excessively, then certain defrag programs claim that they can in addition to defragging, actually compact the MFT by removing old, redundant, obsolete data. This is not technically defragmentation, it is compaction; like the difference between registry defragging and compaction. While this could result in some extra space, it is usually not a whole lot, and more importantly, I have yet to see it actually work (my NTFS volume has an abnormally large 644MB! MFT and I have not been able to shrink it back down with anything).

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There is no risc for you. But the less space is available, the longer will the defragmentation need.

Defragmentation means: re-arrange the data on disk, so that (if possible) all the data in one file sits togetcher, physically on the disk. The if there will be a gain in disk space it will be very, very few bits..

Defragmentation can take a long time, depending on how large the amount of data /disk is and depending on the fragmentation found on the drive. You can stop and restart the defragmentation at any time in most defrag-tools.

Here a visualisation.. Imagine every char in the following ascii chart is a sector on the disk and a

  • "." means: free space
  • "+" means: file A
  • "*" means: file B

Here's your disk:


The system will need to read at several addresses to get the content of file A! After defrag your disk looks like this:


Your system can now read file A or B without readjusting the physical needle on your disk.. so this will be faster.

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I only had defrag data loss one time in my life, it was because I had low disk space and tried to cancel the defrag because it was so slow, unfortunately it corrupted a critical system file, but like I said this only happened to me once in thousands of defrag routines. Defrag does not reclaim any disk space, it just consolidates the files on the hard drive. You need to do a disk clean up to gain disk space.



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As i understand..... Defrag has no risk.Yes it takes hell a lot of time.while you save a file datas are written here and there over the spinning media where it finds empty space.so when you read that particular file data has to retrieved from so many places as it was not written in sequence which makes the program consume more time. By de fragmenting system pick up the fragments and arrange in an order which makes the systems read files faster.Thats it.

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If you find degragmentation to be a problem, you should do one of two things:

  1. Upgrade your operating system. Linux, but also Windows Vista and later don't fragment the disk as much, and Windows after XP automatically defragments whenever usage is low, so it doesn't waste your time.
  2. Get a solid-state disk (SSD). SSD's are truly random-access (like RAM) and don't ever need to be defragged.
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File systems fragment over time. SSD must not be defragmented because this will massively reduce their life time. –  Mike L. Sep 1 '10 at 7:18
Correct. Also, they don't need to be defragged because on non-rotating media a fragmented file and a non-fragmented one can be read in the same amount of time. –  CarlF Sep 1 '10 at 12:35
I'm not sure it's fair to say Vista and 7 fragment less. They do by default, however, defragment regularly of their own accord, which will lead to a perceived smaller amount of fragmentation. That is to say, they don't fragment less, they just defragment more often. –  JBirch Sep 4 '10 at 0:58

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