I encountered a scary looking bug today on a router. I tried to save a config and received a “device busy” error. The issue was a stuck vty connection. After clearing it (clear line vty X) I could wr mem successfully.
https://supportforums.cisco.com/t5/lan-switching-and-routing/4500x-problem-cannot-write-memory-startup-config-file-open/td-p/2603864