It is safe to remove your drive after you have pressed STOP and the light goes out. The fact power is still flowing means the read-head is still out over the platters of the drive when you disconnect it. The sound you hear is the heads parking themselves. There is nothing wrong with that process. What the STOP mechanism does is ensure that all of the files in use have been closed and are not in use when you disconnect the drive, which would otherwise cause data corruption and possibly damage the file system.