Which errors are prevented before a job starts?

Before a transfer starts, Merceq checks whether the source and destination are suitable. For example, it may stop a job or show an explanatory warning if the destination is selected inside the source, the backup disk is the same as the main destination, there isn’t enough free space at the destination, the disk is read-only, or a file larger than 4 GB is being written to FAT32. Protection is used to avoid overwriting an existing destination when filenames conflict.

On macOS, F_FULLFSYNC is used to check that a file has been permanently written to disk. This method may not be supported on some exFAT/FAT or network disks; Merceq uses the standard disk writing method instead and may report weaker durability. The same resistance to power loss isn’t guaranteed for every disk format.