Verify Only is the most misunderstood button in Win32 Disk Imager. Many users assume it repairs a card, speeds up a write, or guarantees a card will boot. It does none of those things. What it does is simpler and still valuable: it reads the destination card and compares it, sector by sector, with the image file, and reports whether they match. That comparison is the closest thing to proof that a write actually landed correctly.
This guide is for anyone who has flashed a card that must work first time: a Pi going into a remote location, a card for a demonstration, a master image for cloning, or any card where a second trip to fix it would be costly. It explains what Verify Only checks, how to run it, how to read its result, and when it is worth the extra time it takes.
Verify Only exists in v1.0.0, the current Win32 Disk Imager release from 2017-03-08. The program is Windows-only and GPLv2 licensed, and Verify Only works the same way regardless of which image you wrote, whether it is Raspberry Pi OS, another Linux image, or a backup you made with Read. It compares data, not bootability, and that distinction matters for interpreting its result.
Before you start
- Verify Only reads the destination, so it does not erase anything. Still, do not remove the card during verification, because an interrupted verify gives no useful result.
- Keep the original .img file. Verify compares the card against that file, so if you delete or move the image, there is nothing to compare against.
- Verify takes roughly as long as a write, because it must read the whole card. Plan for that time when a card must be ready by a deadline.
Understanding what a match means will save you from overconfidence. A verify pass that succeeds means the card’s contents match the image file, sector by sector, for the area the image covers. It does not mean the image itself is correct for your device, that the card is healthy for long-term use, or that the Pi’s power supply is adequate. Those are separate questions, and a verify pass answers only the first one: did the write land correctly?
Step by step
1. Write the image normally first
Flash the card with Win32 Disk Imager as usual: select the .img file, double-check the drive letter, click Write, and wait for success. Verify Only is a separate step after writing, not a replacement for writing. If you run Verify Only on a blank or unrelated card, it will report a mismatch, which tells you nothing except that the card does not contain that image.
2. Select the same image and the same device
Leave the Image File field pointing at the same .img file you just wrote, and leave the Device drop-down on the same drive letter. Verify compares exactly those two things, so selecting a different image or a different card invalidates the comparison. If you ejected the card after writing, reinsert it and confirm the letter again, because drive letters can change when devices are reconnected.
3. Run Verify Only and wait
Click Verify Only and wait for the process to complete without removing the card or letting the computer sleep. The progress bar advances as the card is read and compared. A full verify of a large card takes minutes, and interrupting it forces you to start again. Use that time to prepare the target device, rather than starting another disk-heavy task on the same computer.
4. Read the result correctly
A success message means the card matches the image. A failure message means it does not, and you should reflash the card, preferably after trying a different reader or USB port. If the same card fails verification twice, set it aside and test a different card, because repeated verify failures are a strong sign of failing flash or a faulty reader, not of bad luck.
5. Boot-test after a successful verify
Even after a successful verify, boot the card in its target device before you rely on it, especially for a remote deployment. Verify proves the data matches; booting proves the device can use it. The two tests together give you confidence that neither the write nor the device configuration is at fault.
Frequently asked questions
Does Verify Only change or repair the card?
No. It only reads the card and compares it with the image file. It does not write, repair, or change anything on the card.
Is Verify Only the same as writing again?
No. Writing copies the image to the card; verifying reads the card back and checks it against the image. Verify is a check, not a second write.
Should I verify every card I flash?
Verify when failure would be costly, such as a remote Pi or a master image for cloning. For quick desk tests, a boot test alone is usually enough.
For the full set of guides, see the Tutorials hub, and get the program from the download page. Related guides include: how to use win32 disk imager, clone one sd card to many, how to back up sd card to img — each one covers one job in the same plain style as this guide.