This error (Cannot copy file (87): The parameter is incorrect) most often occurs when a file over 4GB in size is being copied (attempted copy, anyhow) 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.

This error-87 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, flash drives, etcetera) can help narrow down the problem.

Unfortunately, it is a Windows error (error #87, as shown), and it is annoyingly 'unspecific'.