badblocks scans a device for unreadable blocks.

# Non-destructive read-only scan (default, safe)
sudo badblocks -sv /dev/sdX

# Non-destructive read-write scan (backs up and restores each block; slow, still risky on a failing disk)
sudo badblocks -nsv /dev/sdX

# DESTRUCTIVE write test: overwrites the entire device with a random pattern
sudo badblocks -c 10240 -wsvt random /dev/sdX

The -w write-mode test destroys all data on the device. Use it only on empty or new disks. -s shows progress, -v is verbose, -c sets blocks tested at once, -t random selects the pattern.

Modern disks remap bad sectors internally, so badblocks finding errors means the spare pool is nearly exhausted. Check health first with smartmontools (smartctl -a): reallocated and pending sector counts are the better early warning. For large disks add -b 4096 to match the sector size.