This errno, "Device busy", is used as a catch-all for cases when exclusive access to to an object (process, device, etc.) is already held, if you attempt to gain access when the object is busy. This may be write access when something is already open for writing, read access on some devices, attaching (as in, debugging) a process which is already attached to, trying to open a tty with the exclusive flag already set, or any number of other things.