Time Machine Password for external drive is not working

Earlier this week, I connected a new Toshiba Cenvio Flex external, 4TB drive to my iMac, and set it up in Time Machine as my regular, encrypted back up option. The installation appeared to work correctly and the initial back up was performed correctly.

The next day, I replaced the cable connecting the drive. I got a pop up asking for the password for the Toshiba drive. I entered the password I created during set up, but got a pop up that the password entered was incorrect. It is possible that I wrote the password down incorrectly, but I don't think so. I have tried repeatedly but get the same error.

There does not seem to be any option to reset the password.

I decided the best next step was to try and get the new, external drive back to its factory settings (wipe it clean) and reset it up as a recognized drive and the destination for Time Machine.

I deleted the disk from the TimeMachine settings on the iMac (so at this point, Time Machine is not active on the machine).

I still get the same error message when the drive is connected.

I contacted Toshiba, but they indicate this is an Apple software issue and they can provide no assistance.

Any ideas on how to get the iMac to forget this drive was ever connected and start over?