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

If you suspect your SD card may have bad blocks, there are a few ways you can test it to find out. Bad blocks are sections of the memory that can no longer reliably store data due to physical damage or manufacturing defects. Here are some signs of bad blocks and methods to detect them on your SD card.

Signs of Bad Blocks on an SD Card

Here are some common symptoms that may indicate your SD card has developed bad blocks:

  • Frequent read/write errors when trying to access files on the card
  • Certain files can’t be opened or seem corrupted
  • “Card not formatted” error messages, even though the card has already been formatted
  • Hanging or freezing when trying to access the SD card
  • Strange behaviors like filenames changing spontaneously

These issues tend to get worse over time as more blocks go bad. The problems may only affect certain files at first before becoming more widespread. Usually, bad blocks develop after lengthy regular use of the SD card.

Testing for Bad Blocks Using H2testw

One of the best ways to thoroughly test for bad blocks is with a free tool called H2testw. This program performs a full scan of the drive to identify bad sections. Here is how to use H2testw to check an SD card:

  1. Download H2testw for free online. It runs on Windows, Mac, and Linux.
  2. Insert your SD card into your computer’s card reader. Make note of the drive letter assigned to it.
  3. Open H2testw and select your SD card’s drive letter. Click “Write + Verify” to start the test.
  4. H2testw will fill up the entire card with test data then read it back to check for errors.
  5. If any bad blocks are detected, they will be displayed in red in the results.
  6. The test can take a while to fully run depending on the size of your SD card and computer speed.

If H2testw reports bad blocks, it’s highly recommended to stop using that SD card, as the problems will likely get worse. Even a single bad block can potentially corrupt files.

Using Windows’ Chkdsk Utility

Windows has a built-in disk scanning utility called Chkdsk that can check SD cards for bad sectors. To use it:

  1. Open the File Explorer in Windows.
  2. Right click your SD card drive and select “Properties”.
  3. Click the “Tools” tab in the properties window.
  4. Under Error Checking, click “Check” to start scanning the drive.
  5. Make sure the option “Scan for and attempt recovery of bad sectors” is checked.
  6. Click “Start” to begin the test. Chkdsk will try to repair any problems found.

If Chkdsk is unable to fix the bad blocks, it will mark those sectors as ‘damaged’. While the data may still be readable, any damaged blocks indicate the SD card is faulty and nearing the end of its lifespan.

Using fsck with Linux

The fsck tool in Linux provides capabilities similar to Chkdsk for checking storage devices for bad sectors. To run a scan on an SD card:

  1. Open a Terminal window in Linux.
  2. Unmount the SD card by typing umount /dev/sdX# (replace X# with your actual SD card drive).
  3. Run fsck with fsck -f /dev/sdX# to do a forced full check.
  4. Review any errors reported. -a can be added to automatically repair issues if possible.
  5. Remount the card after the check using mount /dev/sdX# /media/SDCARD (replace SDCARD with your mount point).

This will perform a thorough scan of the SD card, marking any bad blocks it finds. While Linux tools like fsck are powerful, third-party tools like H2testw tend to be more user-friendly for beginners.

Checking SD Cards Using macOS Disk Utility

On Mac computers, you can use the built-in Disk Utility program to run a verification test on your SD card.

  1. Insert your SD card into the Mac’s card reader.
  2. Open Disk Utility (located in Applications/Utilities/).
  3. Select your SD card volume on the left side panel.
  4. Click First Aid at the top and choose “Verify Disk”.
  5. Disk Utility will scan the disk for problems and report any issues found.
  6. You can also click “Repair Disk” to attempt fixing errors.

Disk Utility may detect bad sectors and attempt to remap them. However, for more comprehensive testing, a dedicated tool like H2testw is still recommended.

Checking Card Health with S.M.A.R.T.

Some newer SD cards support S.M.A.R.T. health monitoring, which can provide information about the condition of the card. S.M.A.R.T. (Self-Monitoring, Analysis and Reporting Technology) checks various usage metrics and error statistics reported by the device.

Support for reading S.M.A.R.T. data varies across operating systems. On Windows, a utility like HDD Guardian can be used. On Mac, Disk Utility has built-in support for viewing S.M.A.R.T. information. There are also cross-platform GUI and terminal tools available.

High “reallocated sectors count” or raw values in S.M.A.R.T. readings can indicate the presence of bad blocks on the SD card that have been remapped to spare sectors.

Signs It May Be Time to Replace Your Card

While tools can help identify bad blocks, SD cards with a significant number of defective blocks should be replaced. Here are some warning signs your card may be too faulty for further use:

  • 10+ bad blocks reported
  • Frequent read/write errors and data corruption issues
  • Unstable performance and problems detected across multiple OSes/devices
  • S.M.A.R.T. data shows high remapped sector counts or raw values
  • Card has reached its write endurance limit based on age and usage

It’s generally recommended to replace SD cards displaying these types of symptoms. The storage reliability will continue degrading over time. Periodically backing up important data from the card is also a good precaution against data loss.


SD cards can develop bad blocks, but there are various utilities available to detect them on Windows, Mac, Linux and other platforms. H2testw provides the most thorough scan of your entire card to pinpoint problem areas. S.M.A.R.T. support on newer cards can also help identify defects and failures. While tools may be able to remap some bad sectors, multiple bad blocks typically indicate an SD card at the end of its usable life that should be replaced.