Read Only Allocated Partitions is a checkbox in Win32 Disk Imager that changes how Read works. Normally, Read copies the whole card, sector by sector, including empty and unallocated space, and the output is as large as the card. With this option enabled, Read skips unallocated space and copies only the partitions that are actually allocated, which can produce a much smaller .img file. That sounds like a free saving, so it is worth understanding exactly what it does and when it does not help.
This guide is for users who have made a full Read backup, been surprised by its size, and wondered whether this option is the answer. It explains the option in plain terms, shows when it produces a smaller file and when it produces the same size, and warns about the cases where a smaller image is not a faithful backup for your purpose.
The option is part of Win32 Disk Imager v1.0.0, released 2017-03-08, the current Windows-only GPLv2 release. It does not change how Write works, and it does not compress anything. It changes which sectors Read copies, and that distinction determines everything else in this guide.
Before you start
- This option affects Read only. It does not make Write faster, and it does not shrink an existing .img file you already made.
- Always keep a full backup of anything irreplaceable before you experiment with a smaller backup method. Test the smaller image by restoring it to a spare card before you rely on it.
- Understand your partition layout before you enable the option. If the card’s partitions already fill most of the card, the saving will be small, and the extra complexity may not be worth it.
To see why the option sometimes helps a lot and sometimes helps not at all, picture a 64 GB card with a 4 GB system on it and 60 GB of unallocated space. A normal Read copies all 64 GB. With Read Only Allocated Partitions, Read copies roughly the allocated partitions, so the image may be closer to 4 GB plus partition overhead. That is a large saving, and it is the case the option was designed for.
Step by step
1. Inspect the card’s partitions first
On the Pi or in a partition tool, look at how the card is divided. Note the size of each partition and how much unallocated space exists at the end of the card. If unallocated space is a large share of the card, this option is likely to help. If the partitions fill the card, expect only a small saving, and consider compression with 7-Zip instead, which shrinks empty space inside partitions as well.
2. Enable the option before you press Read
In Win32 Disk Imager, tick Read Only Allocated Partitions before starting the Read operation. The option must be set before Read begins; enabling it afterwards does not change an image you already made. Confirm the drive letter and the output file name as usual, because the rest of the Read process is unchanged.
3. Run Read and compare the output size
Click Read and wait for completion, then compare the output file size with the card’s full capacity. A much smaller file, when unallocated space was large, is the expected result. A file close to full size, when partitions fill the card, is also expected, and it does not mean the option malfunctioned. Record the size so you can compare future backups.
4. Test the smaller image by restoring it
Restore the smaller image to a spare card with Write, and boot that card in its target device. Confirm the system works and the data you care about is present. This test is essential, because a smaller image that restores correctly is a valid backup, while a smaller image you have never restored is only a hope. Never test by restoring over your only working original.
5. Decide whether to use it routinely
If the test passes and the saving is significant, you can use the option for routine backups of that card layout. If the saving is small, or if you need a bit-exact copy of the whole device for forensic or diagnostic reasons, stick with a full Read. The right choice depends on your layout and your reason for backing up, not on a general rule.
Frequently asked questions
Does Read Only Allocated Partitions compress the backup?
No. It skips unallocated space outside partitions, which makes the image smaller, but it does not compress anything. Use 7-Zip afterwards if you want further compression.
Why did my image stay large with this option enabled?
Because the card’s partitions already fill most of the card. The option skips unallocated space, not empty space inside a partition, so a full card layout gives little saving.
Is a smaller image safe to restore?
Yes, if you test it by restoring to a spare card and booting it. For normal system restores it is valid, but for forensic copies where unallocated data matters, use a full Read instead.
For the full set of guides, see the Tutorials hub, and get the program from the download page. Related guides include: how to back up sd card to img, compress img file 7zip, restore img backup to sd card — each one covers one job in the same plain style as this guide.