IO usage is not just uploading and downloading. Actually, I don’t think FTP and file manager usage is counted for IO usage at all.
No, IO usage tracks disk interaction from PHP scripts. If you read and write many (large) files, that results in high IO usage.
Unfortunately, we only track IO usage for the account as a whole. There isn’t really a way to easily figure out which request, visitor, script or even domain is responsible for the IO usage on your account.