I'd remove the hard drive from the external enclosure case and attach it locally and remove the partition on that drive, make a new partition, and then format it clean. You then can stuff it back into the external enclosure and have a working external again. Depending on the type of external enclosure this can be eay to remove the hard drive safely or a royal pain. I have a seagate that I needed to perform this on, and it was 2 plastic clam shells snapped together with the drive at the core of it. I had to carefully bend the plastic tabs while also prying the plastic apart at the seam. Got the 500GB out and was then able to connect the SATA HD locally and wipe the partition and create new partition and then format it clean. Then stuff the hard drive back into the external enclosure and test it first before snapping the 2 plastic clam shells together. It worked and so it got snapped back together and no problems.
