I have a 2TB Toshiba I use to clone/back up my Macs to. It has 2 partitions , 1 I use for the iMac (A), the other for my MBP(B). When plugged in, Startup Disk shows both A & B. Restarting with Option Key down, shows both A & B. *--*--*--*--*--*--*--*--*--*--*--*--*--*--*--*--*--*--*--*--*--*--*--*--*--* Ok, so I created the iMac back up by using Carbon Copy Cloner to make an initial clone, on a freshly erased partition. I used Chronosync to update the clone to match the active iMac drive several times before taking the iMac for a few days at the Apple Store for repair of the stand hinge. I had found (before taking it in) that the iMac would not boot from the Toshiba Drive. It would start, show the apple icon, than turn off (just go black). I ASSUMED that this had something to do with the Toshiba's power system. HOWEVER, this (appears) to not be the case: 1) I can boot the iMac using the Macbook Partition. 2) I cloned the imac Clone from the toshiba to a third drive that had previously had a working boot image on it, and got the same results as above (the shut down during start up). The iMac does boot from other USB sources, as said. SO, the question is what went wrong with the clone I made? Something with the combining CCC and Chronosync? (This is all post-mortem...I'm just doing a fresh install on the imac now, but want to know for the future).