Using Old Time Machine with New iMac?

Here's a guess. Look the the name of the folder that holds all the backups. Does it match the name you gave to the new Mac or the name of the old Mac? It should match the new one. This is how TM can keep backups for multiple Macs all of the same drive.

 

I have a similar problem, except that I'm using the same machine.

I had the logic board replaced under warranty, and my old TM backup is not recognized as belonging to my machine. It creates a new backup and I cannot go back in time with the old. My machine is called "thor" and matches the old backup name. But TM created a new backup called "thor 2." I did not use it for a restore since I had a SuperDuper clone to use.

 

This has been discussed in many other threads. Basically, it can't be done. Not that I've read of at least.

The easiest thing to do would be to delete your backup folder and start over.

 

I had the same question, and that is what I had to do in the end (Format my backup drive and start over). I felt terrible deleting months of backups, but I guess its okay since I rarely have to go back and retrieve old files, and at least I had my current home folder transfered from the old computer via. Time Machine before I did the formatting.

It's kind of weird that there is no way to keep backing up to the same Time Machine backup file after using Time Machine to transfer your files to a new computer.