I would say it's safe enough, but if you really want to make sure something's been unmounted, open up Disk Utility, select the appropriate mount point from the list on the left, then click the Unmount button up on top (if it's available). If the mount point doesn't show a device connected, or if the Unmount button is not available, then you can definitely unplug your drive.
Honestly, though, it sounds like it was ejected during your dabblings.
Reformatting drives is quick and easy if you need to do it. Otherwise, be sure to buy drives that are pre-formatted in the future.