If you back up SD cards with Win32 Disk Imager, your backup drive fills up far faster than you expect. That is not a mistake, and it is not because your cards are full. It is because of how the tool works, and once you understand that one fact, choosing a backup drive becomes simple. This guide is for Pi owners, retro-gaming collectors, and anyone who keeps more than one card image.
Verdict up front: buy a drive sized for a library of full-size images, not for a single backup. A 1 TB external drive is a sensible minimum for a handful of cards, and a 2 TB drive is the safer default if you keep several 64 GB or 128 GB card images. A portable hard drive is the best value for a deep archive; an external SSD is the better choice if you restore often or carry the drive with you.
The core fact is this: Win32 Disk Imager Read output size equals the full device size, with no compression. A 128 GB card makes a 128 GB .img file, even if only a few gigabytes of that card hold actual files. A 64 GB card makes a 64 GB file, and a 32 GB card makes a 32 GB file. Back up five 128 GB cards and you already need 640 GB, before a single other file. That maths, not marketing, should set your drive size.
Capacity maths: size for the library
| Cards you back up | Image size each | Library total (uncompressed) |
|---|---|---|
| 4 cards at 32 GB | 32 GB | 128 GB |
| 4 cards at 64 GB | 64 GB | 256 GB |
| 4 cards at 128 GB | 128 GB | 512 GB |
| 8 cards at 128 GB | 128 GB | 1,024 GB |
| 8 cards at 64 GB plus 4 at 128 GB | 64 GB / 128 GB | 1,024 GB |
Compression changes the picture after the fact. Our guide to compressing an .img file with 7-Zip shows how empty space in an image often compresses well, but you should not buy a drive that only fits if every image compresses perfectly. Buy for the uncompressed total, then treat compression as a bonus. The backup workflow itself is covered in backing up an SD card to an .img file, and restoring is covered in restoring an .img backup to an SD card.
Comparison: hard drive versus SSD for .img archives
| Feature | External hard drive | External SSD |
|---|---|---|
| Capacity per unit of money | Higher: best for large libraries | Lower: you pay more per gigabyte |
| Restore speed | Slower, but fine for occasional restores | Faster, noticeable on 128 GB images |
| Durability when carried | Sensitive to drops while spinning | No moving parts, tougher in a bag |
| Best role | Deep archive at home | Working set and portable restores |
| Size guidance | 2 TB is a safe default | 1 TB if you compress and rotate old images |
For a current drive to start from, see this backup drive on Amazon (Sponsored) — check the current price and confirm the capacity, connector, and whether it is a hard drive or an SSD on the product page. See our affiliate disclosure for how sponsored links work on this site.
What to avoid
- Buying exactly one image worth of space: the second backup is when the drive runs out. Add room for at least twice the cards you have today.
- Keeping the only copy on the same card or PC you imaged: a backup on the device it backs up is not a backup. Keep images on a separate drive, and ideally a second copy elsewhere.
- Deleting the original card before verifying the image: verify the image, or at least confirm its size matches the full card size, before you wipe or reuse the source card.
- Forgetting that Write overwrites everything: Win32 Disk Imager Write overwrites the entire destination drive. Restoring an image to the wrong drive destroys whatever was on it, so always check the drive letter first.
A simple archive routine
Name each image with the card, the date, and the project, for example pi-media-2026-10-24.img, and keep a short text note alongside it listing what the image contains. Store images on the backup drive, compress older ones you rarely need, and keep one copy offsite if the images matter — cloud backup for .img files is compared in our pCloud versus Backblaze guide. You can get Win32 Disk Imager from our download page.
Frequently asked questions
How much backup drive space do I need for .img archives?
Size it for a library, not one image. Because Win32 Disk Imager Read output equals the full device size with no compression, a 128 GB card makes a 128 GB .img file, so ten such backups need over a terabyte before you add any other files.
Can I compress .img files to save space?
Yes, after the image is created you can compress it with a tool such as 7-Zip, which often shrinks an image a lot when much of the card is empty. Keep at least one uncompressed copy of any image you may need to restore in a hurry, and see our compression guide for the steps.
Should I use an SSD or a hard drive for .img archives?
A hard drive usually gives more capacity per unit of money, which suits a large archive library. An SSD is faster and tougher when you carry it around, but you pay more per gigabyte, so it is best for a working set rather than a deep archive.