A backup made with Win32 Disk Imager Read is a complete, sector-by-sector copy of an SD card, saved as an .img file on your computer. Unlike copying files in File Explorer, this method also captures the boot sector and partition table. That is why Read is the right backup method when you need to restore a working card exactly as it was.
This guide is for anyone who has a working card they cannot afford to lose: a Raspberry Pi project, a retro console setup, an industrial controller, or a camera card with a custom layout. It walks through the Read operation from start to finish, explains why the output file is so large, and shows how to store the backup so it is actually useful when the original card fails.
Expect one surprise if this is your first Read backup. The .img file will be as large as the whole card, even if the card looks nearly empty in File Explorer. That is not a bug. Read copies every sector, including empty space, and it does not compress anything.
Before you start
- Read does not erase the source card, but do not write to the card while a backup is running. Close programs that might modify it, and remove it from any device that could write logs or data during the copy.
- Make sure the destination drive on your computer has more free space than the full size of the card. A 64 GB card needs more than 64 GB free, plus room for the compressed copy if you plan to make one.
- Use a reliable card reader and do not disturb the card during the read. A loose connection can produce a backup that looks complete but contains read errors.
- Label the backup as soon as it finishes. A file named image.img tells you nothing in six months, while a name with the device, date, and purpose does.
Before you start, decide what you are backing up and why. If the card boots a device, a full Read image is the only backup that restores bootability without rebuilding the system. If you only need photos or documents from a FAT32 card, copying files may be simpler and far smaller. Choosing the right method saves disk space and avoids disappointment later, when you discover a file copy cannot boot a Pi.
Step by step
1. Insert the card and confirm its drive letter
Insert the card into your reader, wait for Windows to recognise it, and note the drive letter in File Explorer. If Windows offers to format the card, choose Cancel. You are reading the card, not repairing it, and formatting would destroy the very data you want to preserve. If the letter is missing, reseat the card or try another reader before continuing.
2. Open Win32 Disk Imager as administrator
Right-click Win32 Disk Imager and choose Run as administrator. Administrator rights are needed to read the raw device, and running without them is a common cause of access errors. The program is Windows-only, and there is no Mac or Linux version, so this guide assumes a Windows computer with a working card reader.
3. Choose an output file name and location
Click the folder icon next to the Image File field, browse to a folder with enough free space, and type a clear file name ending in .img, for example raspberry-pi-home-assistant-2026-10-24.img. Avoid saving the backup onto the same card you are reading from, which the program will not let you do sensibly, and avoid a nearly full drive that could run out of space halfway through.
4. Select the device and press Read
Choose the correct drive letter from the Device drop-down, then click Read. Read copies the whole device sector by sector, so the progress bar advances steadily and the output file grows to the full device size. Do not remove the card or let the computer sleep during the read, because an interrupted read produces a truncated image that may look plausible but will not restore cleanly.
5. Wait for completion and check the file size
Wait for the success message, then check the .img file in File Explorer. Its size should be close to the full capacity of the card, not to the amount of data you think is on it. If the file is much smaller, something went wrong, and you should repeat the read. Note the size and the MD5 or SHA-256 hash if you want a way to check the file later for corruption.
6. Compress and store the backup
Because a raw image includes empty space, it usually compresses very well. Use 7-Zip to compress it, as described in the compression guide, and keep at least two copies in different places. Test the backup only by restoring it to a spare card, never by restoring it over your only working original, because a restore overwrites its destination completely.
Where to keep the backup
A backup only helps if it survives the card it came from, so copy the finished .img file off the computer you flashed from. A dedicated backup drive (Sponsored) works well for large images, and you can also keep a second copy in cloud storage such as pCloud if you want an off-site copy. Label every file with the device, date, and contents before you upload it.
Frequently asked questions
Does Read erase or change the SD card?
No. Read only copies data from the card. It does not write to the card, format it, or change its contents. Still, avoid writing to the card during the backup so the copy is consistent.
Why is my .img file as large as the whole card?
Because Read copies the whole device sector by sector, including empty space, and it does not compress the output. A 32 GB card produces roughly a 32 GB file, which usually compresses well with 7-Zip.
Can I restore this backup to a different card?
Yes, if the new card is at least as large as the original. Cards sold as the same size can differ by a few megabytes, so a restore to a slightly smaller card can fail even when both are labelled 32 GB.
For the full set of guides, see the Tutorials hub, and get the program from the download page. Related guides include: restore img backup to sd card, compress img file 7zip, win32 disk imager allocated only read — each one covers one job in the same plain style as this guide.