Copying a file doesn't guarantee it arrived intact. Bit rot, a flaky USB cable, a failing drive or a dropped network packet can all leave you with a file that looks fine but is subtly corrupted — and most people never find out until it's too late.

This is exactly what CRC (cyclic redundancy check) checksum verification is for. It's a simple, well-established way of proving a file is byte-for-byte identical after a copy, transfer or long-term storage — yet very few people use it, mostly because they've never heard of it.

What a checksum actually does

Any important data should have its CRC value calculated using one of the standard algorithms — typically MD5, SHA1 or SHA256. In simple terms, software totals up all the values inside a file and produces a short text value. That value can be recalculated later and compared to confirm the file is still exactly the same as it was originally.

The best long-term approach is to have the software store these checksum values in a text file, kept either in the same folder as the files you want to protect, or in a master checksum directory at the root of the drive.

Run these checks during any large or important file transfer between devices, and then periodically on anything stored long-term — internal or external HDD/SSD drives, RAID NAS systems, USB sticks, DVD/M-DISC, and even cloud storage.

Two easy methods for CRC checking when copying files

Method 1: verify during the copy

Use software such as SyncBack Pro, which lets you enable Integrity Checking on both the source and destination. When setting up a copy profile, go to the Copy/Delete tab in settings and tick 'Verify that files are copied correctly'. There may be other tools that do the same job, but we've trusted and used SyncBack extensively for many years.

Method 2: generate checksums before you copy

Alternatively, generate a CRC checksum file covering every folder before you move anything. We recommend EF CheckSum Manager — a free, fully working demo is available, and a licence costs around $15.

Using EF CheckSum Manager step by step

Create the checksums
Highlight a folder and choose 'Create CRC Checksum'. Set create to 'One checksum file per folder', select SHA256 as the checksum file type, then tick 'Recurse folders', 'Overwrite existing checksum files', 'Add * to checksum files', 'UNIX format' and 'Verify after creating'. Run it until it has created a .sha plain-text file for every folder.
Double-check before you move anything
Highlight the folder again and choose 'Verify CRC Checksums'. The software lists everything that passed and failed. If anything fails at this stage, you have a separate underlying problem to fix before you copy.
Copy, then verify — don't recreate
Once the files have finished copying to the new storage location, run EF CheckSum Manager again on the new location, but this time choose 'Verify CRC Checksums'. Do not accidentally create fresh checksums instead — that defeats the whole point of the exercise.

If you can't verify it, you can't trust it.

Other checksum tools we tried (and why we didn't stick with them)

We also tested ExactFile, TeraCopy, SFV Checker, MultiHasher, RapidCRC, File Checksum Utility, WinMerge and Multi Commander. None quite made the grade for us compared to the two methods above.

ExactFile in particular was genuinely good, but it's no longer in development, which is why we don't recommend relying on it long term.

What file corruption actually looks like

The video below demonstrates file corruption in practice — the kind of silent error that checksum verification is designed to catch.

Not sure your backups or migrations are actually intact?

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