When is a copy considered complete?

In Merceq’s usual disk-to-disk copy workflow, a file doesn’t immediately appear under its final name. First, a hidden temporary file is created in the destination folder. As Merceq reads the source, it calculates an xxHash64 checksum for the entire file and writes the data to the destination. After writing to the destination disk finishes, it reads the copy from start to finish again and compares the checksums.

The source card or disk is also read again. The checksums calculated from the first source read, the second source read, and the destination copy must match. This also checks for problems such as corrupt or inconsistent data being read during the transfer. This check doesn’t guarantee that the source wasn’t already corrupted during recording; it checks that the transferred content matches the source that was read.

If the check succeeds, the temporary file is given its final name. If the checksums don’t match, the temporary copy is removed, the file is reported as failed, and the source file isn’t changed. Automatic retries after an error aren’t guaranteed; you may need to review the result and start the transfer again.

Copies on the same disk also have their contents compared from start to finish. Files appearing under another name or the copy progress bar reaching the end doesn’t by itself mean verification succeeded.