Why does 6GB content lead to 30 GB *.img?

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pstein
Posts: 8
Joined: Tue Nov 26, 2013 3:42 pm

Why does 6GB content lead to 30 GB *.img?

Post by pstein » Tue Nov 26, 2013 3:51 pm

I have a bootable USB stick with FAT32 file system. When I right-click on the USB stick drive in WinExplorer then it tells me:

Used space ~ 6.8 GB
Free space: ~25.1 GB

Then I start a backup (either *.img or *.imz) which results in both cases in a file archive size of around 31 GB!

How comes?

I would have expected a size of max 6 GB. Considering a compression of the currently uncompressed files
should even yield a size of 4 GB.
Moreover even if I try to compress the *.img archive AFTERWARDS with the well known famous 7zip compressor
I can shrink it to at most 20GB. Why?

It seems to me that USB image tool collects even those sectors which are empty.

How can I tell USB image tool to only include the really, really filled sectors?

Thank you
Peter

Alex
Site Admin
Posts: 235
Joined: Fri Jul 29, 2011 11:59 pm

Re: Why does 6GB content lead to 30 GB *.img?

Post by Alex » Tue Nov 26, 2013 6:39 pm

FAQ:

Q: Why is the image file so large, even if the USB flash drive isn’t full?

A: An image file represents the complete volume on a USB device, also its empty and unused sectors. However you can create a compressed image file, which can reduce the size of an image, depending on how good the whole image can be compressed. Neverheless, if the flash drive has once been filled completely with data, it may be still a large image, because the sectors could just be marked as unused, but still contain the data. This is how the common used FAT file system works. USB Image Tool has no knowledge of the file organisation on a volume and just knows about volumes and devices.

XP1
Posts: 2
Joined: Sat Jan 25, 2014 11:25 pm

Re: Why does 6GB content lead to 30 GB *.img?

Post by XP1 » Sat Jan 25, 2014 11:36 pm

pstein wrote:I have a bootable USB stick with FAT32 file system. When I right-click on the USB stick drive in WinExplorer then it tells me:

Used space ~ 6.8 GB
Free space: ~25.1 GB

Then I start a backup (either *.img or *.imz) which results in both cases in a file archive size of around 31 GB!

How comes?

I would have expected a size of max 6 GB. Considering a compression of the currently uncompressed files
should even yield a size of 4 GB.
Moreover even if I try to compress the *.img archive AFTERWARDS with the well known famous 7zip compressor
I can shrink it to at most 20GB. Why?

It seems to me that USB image tool collects even those sectors which are empty.

How can I tell USB image tool to only include the really, really filled sectors?

Thank you
Peter
You probably have deleted data occupying the free space.

If the unused space were truly empty (all zeros), then the archiver should have compressed the file to approximately the amount of used space or less.

If you want a smaller image file size, then the first thing you must do is do a full format (not quick format) of all zeros. After the full format, you copy your files to the drive and then image your USB flash drive.

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