I have a Western Digital external hard drive that I use to back up my MacBook Pro at my office, using Time Machine to do so. I now want to use the same external hard drive to back up my iMac at home but when I plug in the USB cable to my iMac, the drive does not mount on my desktop and I get the following message "You have inserted a disk containing no volumes that Mac OSX can read." I hesitate to click the initialize button at the bottom of the message for fear of wiping the external HD clean of my backed up MacBook files. I run 10.5.2 on my laptop and 10.3.9 on my iMac. Suggestions to get it recognized on my iMac? Thanks.