Hi, I have a Lacie 500 GB external HD. I just switched from PC to mac and I wan tot reformat my HD so that it is compatible with mac. I went to disk utilities and followed the instructions but when I go to erase it (I choose Mac OS extended journaled) it says: Volume Erase failed volume erase failed with error: could not unmount disk. I can't delete things off my HD and I want to so I can make more space! How the heck do I fix this? thanks!