Why Does the Partition Size Matter Inside an Image File?

This topic is brought up today because we hear some users have issues understanding this point. The partition size inside an image file does matter. The question we will answer today is why it matters, and why an image can fail even when the IMG file itself looks small.

Let us start off with two simplified overviews. First, all storage devices use a partition to define their characteristics. A storage device has a file system like FAT32, exFAT, or NTFS, and that file system has a defined size or digital capacity. These characteristics, along with a few others, are laid out in the partition table.

Second, an image file is the above partition with all its detail — the file system, defined capacity, along with all the actual files and folders on that partition — captured into a single file, typically an .img file.

For a non-technical person, let me use a puzzle as an example.

• The puzzle box is equivalent to the physical device.
• The plastic bag inside the puzzle box, holding all the pieces, is the image file.
• The print on the puzzle box indicating the number of pieces is the partition.
• The puzzle pieces inside the box are equivalent to the data.

At this point we know the image file (.img) is the bag that holds all the puzzle pieces and the data are the pieces themselves. So let’s address the core question: why does the partition size matter inside an image file?

Back to the puzzle box. The outside of the box lists how many pieces are inside. If the physical box size is, say, 8 by 11 inches, it’s logical that a 1,000-piece puzzle would fit. It’s also logical that a 20-piece puzzle would fit. But could a 5,000-piece puzzle fit inside that same box?

From those scenarios, one clearly doesn’t work. The box claims more pieces than the physical space can support. Partitions behave the same way.

Said another way, you cannot use a partition table size of 4GB and try to write that image to a USB stick that has only 1GB of physical storage. Even if the image file itself contains only 1GB of actual data, the declared partition size still must fit on the device.

Here is a real-world example: you can download this IMG file which is only 40MB in size. The IMG contains a 4GB partition. Write it to a 4GB (or larger) flash drive and it works. Try writing it to a 2GB drive and it will not.

What happens?

Windows is smart. All modern versions of Windows compare the partition table size against the available physical memory on the device. If Windows sees the partition claims more space than physically exists, it blocks access to the device and only allows formatting. Formatting rewrites the partition table to match the real capacity.

This behavior exists to prevent fraud. Older operating systems like Windows XP did not validate partition size versus physical memory, which allowed fake-capacity USB drives to be sold. Windows 7 and newer closed that loophole.

This same partition-size rule shows up frequently when creating bootable media. We’ve seen it come into play when users attempt to write boot images that fail silently or appear corrupted, such as when creating tools like the Ultimate Boot CD on a USB flash drive, where the target USB device must meet or exceed the image’s declared partition size.

If you found this article because an image file is not working when written to a USB flash drive, check the partition size inside the image against the physical capacity of the device. The easiest way to verify this is to mount the IMG file and check its properties. Keep in mind the default Windows mount option does not work for this; you need something like this.