Certain files fail to back up with "Destination not available" error

Hello, I have a 320GB My Passport USB drive and for some strange reason a handful of MP3 files get Error Code #12 (The directory or file cannot be created) yet I have around 55 GB free. The backup plan for this has already backed up around 13,000 files atop other plans working without issue, what is causing this?

I also ended up manually copying the offending files to the backup directory but I’m not fond of doing it all the time if this persists with additional files.

Hi, what backup program are you using?

I am using WD Anywhere Backup 4.50.6554, what else do you need to know? I’m also using Windows 7 Home Premium 64 bit.