How much space should I give Time Machine on the partitioned external drive? I have 512G SSD internal on the imac. I was originally thinking I just needed a 3T external drive. But now I have read it may be best to partition the drive and give a portion strictly to time machine. If I only need to give it 500G then I could get away with 3T drive. But if I should give it 1T then I would need to go to 4T drive. Thanks!