I have a 22 gig XP backup file (.bkf) on a usb external drive. Windows disk defragmenter tells me the external volume should be defragmented.

Should I spend the time and effort to defragment the external drive and backup file using a utility like MyDefrag Or, just leave the drive and the backup file fragmented?

  • I wouldn't worry about it either way. – emgee Oct 17 '09 at 18:35

There is no point in defragmenting the backup file, since restoring the backup is not something that one does all day long.

In any case, there's much less reason to defragment an external drive than an internal one.

So my advice is to ignore Windows on this point and leave this drive alone.


To properly understand this you first have to understand fragmentation.

Your hard drive is a magnetic disk that can store binary on/off (magnetic/not magnetic) data in every "track" around the disk (imagine a record, it reads out from the center)

When you save a file to disk, the OS finds a spot for it, and saves it there, so for example if you had 3 text files, 1.txt, 2.txt, and 3.txt, they could be on disk as:


Now, if you wanted to save 4.txt, you'd have to do it between 2 and 3 - there is no room between 1 and 2 (perhaps a file that only took up 2 =s there was deleted, leaving a gap), so your new disk looks like


Neat, right? Not really, how are you going to save 5.txt?!


Eugh! That takes a while to read, doesn't it? It's the same for your hard drive, the read head has to skip over 2.txt AND 4.txt while it's reading it! That's much slower! Now, most hard drives are big enough to store more than 5 text files, so you have a lot more space to play with, but you're also saving much larger files, so it sort of balances out.

In your context, a backup file is MORE likely to get damaged by disk failure, as it's spread out across the disk, so a failed bit is much more likely to contain a bit of your backup. Defragmenting doesn't change the file as the OS sees it, it just sticks it together on the disk, meaning reading and writing it is much faster.


External drive has Mac files on it saved for 7 years. All was well until yesterday I introduced another Mac save trying to place a disk image upon the external backup device, and the disk image was too large, so think perhaps some files were moved or scattered as you described great above. Neither of my 2 Macs (G4 powerbook 17" Mac laptop from 2004 with 1.5GHz processor, and iMac G5 desktop all-in-one screen 1.83Ghz) can read the backup device and its icon doesn't appear on the desktop when plugged in. Same with disk utility not recogizing this suddenly. Then I tried putting the USB into the Dell PC 3GHz (2006) desktop, and it recognized the Mac files to my surprise. I'm now thinking of doing a defrag, so is this ok. Just trying to get things back to normal so my macs can save data back upon the external HD made by western digital (200GB from about 2006 or so). Thought once you have formatted an external backup drive with apple files then that's it and a PC can't do any saves. This is older tech keep in mind now backup drives have dual capability. My device does say "Combo", so maybe I'm in luck? Can the PC resave from this device and back out the other usb port and into a new memory device (like a 500GB new drive that I will run to the store and buy today?), and will it be able to be read as they were saved originally by the 2 Macs?

Why the Macs won't read is either because I had to pull the plug during a disk image save, and/or things got fragmented as per the discussion above... trying not to panic but this is 8yrs and 25000 hours of research on it (too tired from 24/7 attempts to cry)


It should be OK to defragment. The only time files break is when the program that wrote them records where it wrote them and checks that they're still in the same place -- a defragmentation can move files around, so our hypothetical program wouldn't accept the files as genuine.

This is not a technique that's in much use in modern Windows systems, so you're probably OK. If your backup program would function correctly with a copy of that file elsewhere, you can defragment it with no worries.

  • Thanks ~quack, would the bkf file (if needed) be just as good if it were left in the current fragmented state? – Robert Oct 17 '09 at 18:05

Defragmentation really only benefits you if you're accessing the filesystem a lot -- you can read a file off the disk faster if the drive does not have to seek all over the drive. If this is a backup drive, and you access it once a month or the like, I really wouldn't worry about fragmentation.

Defragmenting the drive should not introduce problems, so there should be no harm in it.


If performance is not the issue, defragmentation is not neccesarily bad. It just means that the file is not all in one place, but scattered around the disk.

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