Hi.. I just got an iMac a month back and I would like to use the Time Machine feature on iMac, which takes weekly backups for me to one of my External Hardrive, which I have. Its Western Digital MyBook Essentials Edition 500GB. My imac also has 500GB HDD. So, here are my questions : 1. Can I use this external HDD as a backup drive for Time Machine ? 2. So, it has to be connected on USB and selected as destination on Time machine ? 3. Both are 500GB, so any problems ? 4. Lets say, now I connected my drive to imac and I took a backup. Then next week, when it automatically backs up, then will it overwrite over that old backed up data or write new backup again ? Im asking this, bcz I have 400GB of data on my imac , so if it writes a new one, then from where the space will come in the ext HDD ? 5. Do I need to keep the external HDD always connected to electricity and iMac or I can on it anytime ? How will Time Machine detect it or search for it, in case its not connected ? 6. I dont want to waste money on a Time Capsule, so please help me getting this existing ext HDD to work to solve my problems. Thanks.