NOTE: I misspoke with the smaller drive size....it's 320GB and I've corrected the post. This is the drive that has always contained my Time Machine backup.
Two thoughts: first, is each Time Machine drive trying to back up the full 1 TB internal drive and not just the one partition?
No.
Second, are the TM drives empty or do they contain the previous backups?
No. I used Disk Utility to reformat but Time Machine won't put 250GB of data onto a 320GB drive..... even though it always did so before. It now says: "Time Machine backup needs 505.21 GB but only 319.30 GB are available."