I used my new WD 300G external hard drive to copy some files from my brother's Macbook Pro. After Copying the files to my Macbook Pro, I wanted to use the external drive as my "Time Machine". The time machine prompted me that I need to erase all the files using disk utility. The file structure displayed had the name of the device then a sub disk. I used the erase menu to clear the sub disk. Now when I plug in the external hard drive all I get is a prompt saying the device is not readable by this computer. The same prompt comes up on my brothers computer. Did I ruin the hard drive?