How can I test a CF or SD card for reliability and performance before a shoot?

Flash cards don’t behave like hard drives, so a classic “bad sector” scan is only partly meaningful. The card’s internal controller handles wear leveling and remaps failing blocks, so you usually can’t see the physical flash layout directly.

The most practical test is a full-capacity write/read verification: fill the card completely, then read it back and compare the data. Repeating this with different data patterns can increase confidence. On Linux, badblocks is the tool commonly suggested for this kind of test. A macOS port may be available via ext2-related packages, but results still won’t be truly conclusive.

Important limitations:

• The controller may hide faulty regions by remapping them.
• A card can still fail later even if it passes today.
• Testing itself can’t guarantee future reliability.

So: yes, you can stress-test a card and confirm that its full reported capacity can be written and read correctly, but no software can absolutely prove a flash card is “safe.” For important shoots, the best practice is to test the card in-camera, verify files, and avoid trusting a single card without backup.