I'm not sure what you did, so here's my suggestion:
1. Open Disk Utility in your Utilities folder.
2. After DU loads select your TM backup drive (this is the entry with the mfgr.'s ID and size) from the left side list. Click on the Partition tab in the DU main window.
3. Under the Volume Scheme heading set the number of partitions from the drop down menu to one. Set the format type to Mac OS Extended (Journaled.) Click on the Options button, set the partition scheme to GUID (for Intel Macs) or APM (for PPC Macs) then click on the OK button. Click on the Partition button and wait until the process has completed.
Open TM preferences, select the newly erased backup drive as the TM backup drive, turn on TM.