I have a 24" mid 2007 iMac OS x 10.5. Recently it had a SMART failure so of course I backed up using a 1TB external drive that I formatted w/2 partitions. On one I backed up using Time Machine and on the other I backed up using Tri-Backup 5. Bought a 2TB Seagate HD and swapped out the bad drive. Everything went well. I booted up from the external using the Tri-Backup. So far, so good! I then formatted the new internal using Disk Utility. Named the HD Macintosh HD (just in case). Did a verify permissions on it but it had a lot of differences so I repaired permissions. Now everything looks fine, right? Wrong! At this point, I restarted pressing the option key and selected the new Macintosh HD. I got the Apple, the spinning wheel...then it turned off! When I restart using the external drive, everything is fine! So I restarted pressing "D" and did a hardware check...all is good!!! I don't have the Install Disks to this computer. I bought it used from someone who didn't have them. What do I do?