How do I know if my SD card is bad for blocks?

SD cards have become an essential means of portable storage for many devices, including cameras, phones, and other electronics. However, like any storage medium, SD cards can sometimes fail or become corrupted. One of the more frustrating SD card issues is when the card starts to go bad in terms of blocks – where some blocks of data on the card become inaccessible or unwritable.

What does it mean for an SD card to go bad for blocks?

SD cards are divided into many small storage units called blocks. When you save data to the card, it gets written to these blocks. A block is basically the smallest unit of storage space on the card.

If an SD card starts to go bad in terms of blocks, it means that some of these blocks have gone faulty and can no longer be written to or read from properly. So while most of the card may still work fine, certain blocks are damaged or corrupted.

This can manifest in different ways when using the card. For example, you may find that some files can no longer be accessed, or that the card will reject new data being written to it, or the card may even appear to have less capacity than it should.

Some common signs that your SD card is going bad for blocks include:

  • Unexpected error messages when trying to access files on the card
  • Files disappearing from the card
  • The card reporting less free space than expected
  • The card becoming read-only
  • Difficulty saving new files to the card
  • “Card cannot be accessed” type error messages

So in summary, a card that is bad for blocks is one that is gradually developing more and more unusable or unstable blocks, leading to performance issues and data errors.

How can I test for bad blocks on my SD card?

There are a few ways you can scan for and detect bad blocks on an SD card:

Bad block testing software

Specialty software exists for testing storage devices for bad blocks. These programs will thoroughly scan your SD card, writing test patterns to each block and reading them back to verify consistency and reliability.

Some examples of bad block testing software include:

  • Hdat2 (Windows)
  • Victoria HDD/SSD (Windows)
  • Badblocks (Linux)
  • F3 (Linux)

Using software like this and running an extensive scan of your SD card can definitively identify any bad blocks that may exist.

In-camera bad block detection

Many digital cameras have built-in utilities for detecting bad blocks on SD cards. This allows you to initiate a scan right from your camera. The process and options vary between camera models, but look for your memory card tools under settings or maintenance menus.

Running the camera’s diagnostics will check the SD card for any bad blocks. However, the scan may not be as extensive as with dedicated software.

Using a computer

You can also use a computer to check for SD card errors that may indicate bad blocks. Plug your card into a card reader connected to your computer. You can then run the native error checking utility on your operating system.

On Windows, right click on the card’s drive and select “Properties”>”Tools”>”Check”. On Mac, open Disk Utility and run First Aid on the SD card.

The computer will scan the card for problems like bad sectors. However, this process focuses on the filesystem level, rather than testing every block.

Online bad block checker tools

Some free websites offer an online checker tool for testing storage devices like SD cards and USB drives for bad blocks. These perform read/write tests on your card through your internet browser.

To use them, you need to upload a small test file (e.g. a few MB) to the site. It will then scan this file for integrity to identify bad blocks.

Examples include:

These online tools offer a quick way to do an initial surface-level check for bad blocks. But for deeper and more rigorous testing, use software designed specifically for bad block detection.

How can I recover data from a card with bad blocks?

If your SD card has developed bad blocks, you may be able to recover data that was stored on the affected portions of the card using data recovery software.

When a card starts going bad, your files don’t necessarily disappear right away. Even with bad blocks, the data may still physically reside on the card, but the filesystem has trouble accessing it. So recovery software can scan the raw memory and extract data from the bad areas.

Some top data recovery apps to try for bad block SD cards include:

  • Recuva
  • EaseUS Data Recovery Wizard
  • Stellar Data Recovery
  • Disk Drill

To recover data using one of these programs:

  1. Scan the SD card to see what files can be retrieved
  2. Select the desired recoverable files to restore
  3. Choose a new location to save the recovered files to (not the original bad card)

Depending on the severity of the bad blocks, you may be able to recover some or potentially all data that way. Just make sure to save the files to a different healthy device.

Can I fix an SD card with bad blocks?

Strictly speaking, there is no way to truly fix bad blocks that have developed on an SD card. The physical damage to those blocks on the flash memory remains.

However, you can sometimes revive usability and extend the life of a failing card using software tools for marking bad blocks. This essentially isolates the bad blocks so they are no longer used.

Low-level formatting tools like HDD Low Level Format Tool can identify bad sectors on an SD card. It will then map those sectors as “bad”, allowing the remaining storage space to be accessed normally again.

This prevents data loss from continuing to happen on the bad blocks. But it comes at the cost of losing some capacity, since the marked blocks can’t store data anymore.

You can also use the “full format” option when reformatting SD cards in your computer or camera, as this scans for bad sectors.

However, low-level tools will be more rigorous at detecting and marking bad blocks. Either way, be sure to back up important data first before attempting any fix.

When should I replace my SD card due to bad blocks?

If your SD card is showing signs of bad blocks, it’s unfortunately on its way out. You’ll want to retire it and replace it sooner rather than later.

As a general rule, if more than 5-10% of the total blocks go bad, it’s time to replace the card. The problem will likely continue to get worse, and the card runs greater and greater risk of failure.

Also replace it right away if a critical number of blocks are damaged. For example, if the bad blocks include important metadata like the formatting information, the card may stop working entirely.

Finally, if you do successfully mark the existing bad blocks but then get repeat instances of new blocks going bad, that indicates an unreliable card that needs replacement.

When to keep using vs. replace a card with some bad blocks:

Keep Using Time to Replace
– Less than 5% bad blocks – More than 10% bad blocks
– Bad blocks successfully marked in low-level format – Critical system data blocks affected
– No repeat bad blocks appearing – Repeat bad blocks despite low-level format

How can I prevent bad blocks on an SD card?

To help avoid developing bad blocks in the first place:

  • Buy your SD cards only from reputable brands. Avoid cheap low-quality cards.
  • Handle the cards gently. Drops or impacts can damage the card.
  • Store cards properly in a case when not in use.
  • Don’t expose cards to moisture or extreme temperatures.
  • Safely eject the card before removing it from devices.
  • Be aware that very high resolution cameras or devices may wear cards faster.
  • Reformat your cards every so often after backing up the data.

Following good usage and handling practices will maximize your SD card’s lifespan and minimize the chances of bad blocks occurring.


Bad blocks on an SD card can lead to a number of performance issues and data errors as more blocks fail over time. Using software tools to detect and mark existing bad blocks may help extend the card’s useful life temporarily. But ultimately replacement will be needed for cards with a significant number of unrecoverable blocks.

To avoid bad block problems, invest in high-end SD cards, handle them with care, and reformat periodically. And back up your data regularly so it can be restored if needed after bad block damage.