Switching Time Machine to a new iMac I used Time Machine to restore the hard drive of the old MacBook to my new iMac. I now have what seemed to be two back up files with the same name except the new one ends with the number 2.  This morning I got a message saying I don't have enough room on my 2T external drive to back up the 1T iMac with 264 GB available.
The MacBookPro was also a 1T drive with 90 GB available.  

That is a BIG difference. It seems TM didn't migrate all my data. 
And how is it possible that the 2T drive used for TM can't back up this new iMac?

What is going on and what can I do to get TM to just continue the backup. It is the same exact data just on a new computer and a 2T ext HD should be adequate.

And why the discrepancy in the amount of space available on the two computers when it is supposed to be a complete restore of the data?

Please help!
Try formatting your Time Machine drive/partition and then add it back to TM. Time Machine's backups are device-specific so you can't overwrite/continue using a backup on another machine. 

You could also try deleting your current backups manually, and then clean the .Trashes folder, however formatting is faster. If emptying the trash doesn't help, try cd /Volumes/drivename and then sudo rm -rf .Trashes