Hi, my previous Iomega external drive that i used to back up Time Machine on my iMac has packed in and i need a replacement that would also be suitable to back up my pictures in Photos, something i am yet to do. What sort of spec should i look for and what to avoid ? Should it have it's own mains power supply or run from my iMac and what difference does that make ? My iMac's hard drive is 500gb. Apologies if this is a common ? I did try a search but didn't unearth anything. Thanks in advance for any responses.