Please help! Used Simple Copy, 535 TB of files, 99% finish, then error code 55

I was making a simple backup, Windows 8.1, default settings for Comodo were in place, latest version of Comodo. File size I am backing up is around 535 GB. It got to 99% done, but had error code 55 (access?) and yet when I look at the backup, it looks as though all files are there. The ‘size’ is the EXACT same, but the ‘size on disc’ shows that the new one has 0.15 MB more. The number of folders and files is the same as well, as the original drive I was making a backup from. Does anyone know what could have caused this error? I don’t think I have Comodo set to run as administrator, I could try that next time. I was backing up from external drive, to an external drive. Is there a way to verify that ALL data was successfully transferred? When I go back into Comodo, it shows me that backup was NOT completed successfully, but it looks like everything is there. :o If anyone can help, I appreciate it, thanks so much!