Ran into a situation today where the Finder wouldn’t allow me to delete files from an external hard drive. I had finished working on my podcast project and was in the process of deleting various audio tracks I no longer needed. Pressing Command-Delete elicited an unexpected error -1407 message.
I had quit both of my active audio applications (Soundtrack Pro and iTunes) prior to attempting the file deletions. Everything else about my Mac was operating normally. Using resources at my disposal, I learned that a -1407 error indicated that the Finder was expecting folders and found files, instead. Why the OS thought this is unclear. Each file I was trying to delete was either an .mp3 or an .aif.
Undaunted, I performed the usual type of remedy for problems such as this: I rebooted. After doing so, I was able to trash the files from the hard drive.