The error Cannot copy file (87): The parameter is incorrect most often occurs when a file over 4GB in size is being copied from an NTFS or ReFS partition to a FAT32 partition. The error is because the maximum size of a file that can be stored on FAT32 is 4GB. This is a restriction of the file system and not SyncBack. The only solution is to backup to a file system that can stores files of that size, e.g. NTFS or ReFS.

The can also occur when copying files to a SharePoint server and the file is of a restricted type. For example, a SharePoint server may be configured not to allow .PST files to be copied to it.

If neither of these apply, it is likely caused by hardware errors and/or disk/file errors (corrupt data). Checking if you can copy the same data via Windows (drag & drop, etc), or changing devices (testing with different external USB drives, change the USB cable, flash drives, etcetera) can help narrow down the problem.

Unfortunately, it is generic Windows error that can be returned for numerous undocumented reasons.