Getting Time Machine to recognise a new disk

I am running an iMac with Tahoe 26.1. I want to replace an external HDD with a new Seagate. Tahoe sees the new disk and behaves normally with it, but no matter what I do I cannot get Time Machine (TM) to see it. TM continues to show the last detail of the old disk complete with a red info flag even tho' the old disk is absent. I have rebooted, plugged the new disk directly into the iMac, swapped USB cables, swapped USB hub, erased the new disk several times, used different Apple disk formats all to no avail. I have run out of ideas - any offers?