You use Read Only Allocated Partitions to back up a card, expecting a smaller image that still contains your data, and the result is a few kilobytes, a few megabytes, or an image that appears empty when you inspect it. That is alarming, and it is a user-reported behaviour of this option: on some cards and partition layouts, the allocated-partitions read does not capture what users expect. This guide explains what the option is supposed to do, why it can go wrong, and how to get a backup you can actually trust.
Normally, Win32 Disk Imager’s Read copies the whole device sector by sector, and the output size equals the full device size with no compression. That is predictable: a 32 GB card produces a 32 GB image, even if only 2 GB is used. Read Only Allocated Partitions is meant to avoid that waste by reading only allocated partition areas. In practice, users have reported that it can produce an empty or tiny image, particularly when the partition table is unusual, when partitions are not recognised as allocated in the way the program expects, or when the card’s layout includes partitions Windows does not mount. These are user-reported issues, not certified bugs, and the exact trigger varies by card — which is why verification, not assumption, is the remedy.
Why the image comes out empty or tiny
- Partition table not interpreted as expected. If the program does not recognise the partitions as allocated, it may read little or nothing.
- Linux or unusual partitions. Cards with Linux partitions, multiple small partitions, or non-standard layouts are more likely to confuse the allocated-only logic than a single FAT partition.
- Card not cleanly recognised. A card that Windows sees only partially, or a reader that drops during the read, can truncate the allocated calculation.
- Wrong device selected. Reading the wrong, smaller device — for example a RAM disk or a small partition — produces a small image that looks empty compared with expectations. Always confirm the device size before reading.
Fixes, cheapest and quickest first
- Check the image size immediately after the read. Compare it with the used space on the card. If it is orders of magnitude smaller and you expected data, do not trust it — treat it as a failed backup until proven otherwise.
- Do a full-device Read instead. Uncheck Read Only Allocated Partitions and read the whole device. The file will be large — equal to the full device size — but it is the predictable, complete copy. See allocated only read and how to back up an SD card to an image.
- Compress the full image afterwards. A full image compresses well when much of the card is empty. Use 7-Zip or a similar tool on the .img file — see compress an image with 7-Zip — rather than relying on allocated-only reading to save space.
- Test-restore before you depend on any image. Write the image to a spare card and confirm the device boots or the files are present. An untested backup is not a backup.
- Re-read with a different reader if the full read also looks wrong. A flaky reader can truncate reads, so a second reader is a cheap diagnostic before blaming the option itself.
The practical rule is simple: use allocated partitions only as an experiment, never as your sole backup for irreplaceable data, until you have test-restored an allocated image from that exact card layout and confirmed it works. For anything important, the full read plus compression route is slower and larger on disk, but it removes the uncertainty. Get a fresh copy from the download page if needed, and see the troubleshooting hub for related read problems.
When to stop and switch tools
Stop using the allocated-partitions option for that card if it repeatedly produces tiny or empty images while a full read produces a correct, restorable image. That tells you the option and that card’s layout do not get along, and retrying it will not change the layout. Switch to full reads plus compression for that card. If even a full read produces a tiny or corrupt image, stop and suspect the reader or the card itself — see Error 121 for the hardware checks — because no read option can fix a device that cannot be read reliably.
FAQ
What does Read Only Allocated Partitions actually do?
It is intended to read only the parts of the device that belong to allocated partitions, rather than the whole device, so the resulting image should be smaller than a full-device copy.
Why is my allocated-partitions image tiny or empty?
User reports describe cases where the option produces a much smaller image than expected, or one that appears empty, often linked to how the program interprets the partition table on that particular card.
Should I use allocated partitions for backups I depend on?
Only after verifying the image restores correctly. For a backup you cannot afford to lose, a full-device Read that you have test-restored is the safer choice until you have proven the smaller image works.