A cyclic redundancy check (CRC) error means the computer read data off the drive and it didn’t match the checksum stored with it — so something is wrong with the data, or the disk beneath it. Sometimes it’s a one-off; often it’s bad sectors on a drive that’s starting to fail. Here’s what it means, what not to do, and how the data is recovered.
A CRC is a maths check that data hasn’t changed since it was written. When it fails, the bytes coming off the disk are wrong — usually bad sectors, a failing drive, or a flaky cable. Copying stops, and forcing it can make a failing drive worse.
chkdsk /f forces a failing drive to re-read and remap bad areas, which stresses a disk that’s already struggling and can turn a recoverable drive into an unrecoverable one. If the data matters, image it first.When data is written to a disk, a small check value — the cyclic redundancy check — is stored alongside it. Every time that data is read back, the drive recomputes the check and compares. “Data error (cyclic redundancy check)” simply means the two didn’t match: the bytes coming off the disk are not the bytes that were written. It shows up on hard drives, external drives, USB sticks, SD cards and optical media alike. It doesn’t always mean the whole drive is lost — but it is a clear sign that something physical is wrong, and it should be treated with care rather than repeated retries.
The commonest cause is bad sectors — areas of the disk the drive can no longer read reliably, which multiply as a drive ages or begins to fail. A loose or failing cable can also do it, and on an external drive the USB bridge inside the enclosure is as likely a culprit as the disk, so it’s worth trying a different cable and port first. Less often, the fault is file-system corruption from an improper ejection or a power cut during a write. The pattern tells you a lot: a single CRC error that clears is usually minor; one that keeps returning, or spreads to more files, points at a disk that’s deteriorating. On NTFS, that same bad-sector damage often surfaces as the “file record segment is unreadable” error during a chkdsk scan.
Windows will often suggest running chkdsk /f /r, and on a healthy drive with minor corruption that’s a reasonable fix. On a failing drive it’s the wrong move. chkdsk forces the drive to re-read every sector and remap the bad ones, hammering a disk that’s already on the edge — and in the process it can overwrite the very data you’re trying to save. If you don’t know whether the drive is sound, don’t run it. Copy off what you can, and the moment copying throws CRC errors or slows to a crawl, stop and treat the drive as failing.
The safe route is the opposite of forcing it. The drive is imaged read-only, retrying and skipping the bad sectors so the good data comes off without stressing the disk any further, and the files are then rebuilt from that image rather than from the failing drive. A single hard drive is from £300 +VAT; an external drive is sent whole, since on a WD the encryption key lives on the bridge board and the bare disk is no use on its own; a memory card or USB stick is from £250 +VAT. Every job starts with a free diagnostic and a fixed written quote.
A cyclic redundancy check (CRC) is a maths check stored with your data to confirm it hasn’t changed. A CRC error means the data read back off the disk didn’t match that check — so the bytes are corrupt at the physical level. It’s most often bad sectors on a drive that’s starting to fail, but a loose cable or file-system corruption can also trigger it.
Only if you’re confident the drive is healthy. On a failing drive, chkdsk /f /r forces it to re-read and remap every bad sector, which stresses a disk that’s already struggling and can overwrite recoverable data. If the files matter and you don’t know the drive is sound, don’t run it — image the drive first.
Usually, yes. The drive is imaged read-only, retrying and skipping the bad sectors so the good data comes off without stressing the disk further, then the files are rebuilt from that image. A single drive is from £300 +VAT after a free diagnostic.
Often the same bad sectors, but on an external it can also be the USB bridge in the enclosure or a failing cable rather than the disk itself. Try a different cable and port first. If it persists, send the whole unit — on a WD drive the encryption key lives on the bridge board, so the bare disk is no use on its own.
Not always, but it’s a warning sign. A one-off CRC error can be corruption or a cable; one that keeps returning or spreading usually means bad sectors on a failing drive. If it’s also clicking or slow, treat it as a failing drive and recover the data before doing anything else.
If the error keeps returning or the drive is slow or clicking, don’t run repeated repairs over it. Send it in and we’ll image it read-only past the bad sectors — free diagnostic, fixed quote before any work.