copying folders with files to external HD

when I do, sometimes during the copy over, I get an error message;

Sorry, the operation could not be completed because an unexpected error occured (Error code -50).

Any reason this might be?

Is your external drive formatted as FAT32 (MSDOS)? If so, it can not handle any files larger than 4 GB, so if you have any large files it would cause an error. If you don't need to use that drive with a Windows computer, than format it as Mac OS Extended (Journaled) using Disk Utility.

ok

thanks ADKhikerVW. much appreciated.