Cannot copy file (5): Access is denied on certain files

I have been happily using SB for years, but recently have been having a problem.

I always get a message: "Failed to copy from Source : Safe copy failed. Failed to rename temp file: Cannot copy file (5): Access is denied" for a dozen or so files. They aren't anything unusual, like system files or anything, just regular MS Office documents, etc.

I am backing up to an external hard drive via USB2.

I updated to v6, but get the exact same results.

As an experiment I backed up the entire folder to another folder on the computer HD, instead of the external, and it worked perfectly.

Any suggestions?

Thanks,

David