back up

With the drive in question directly attached to the computer

Open Disk Utilities

1 - It will present options >> Disk Utilities >> View >> View ALL attached Drives. 

2 - Choose the Upper Most Drive ( not the volumes indented and list below ).

3 - Look for the Name of the External Drive - example Samsung, Western Digital, etc.

4 - That is the drive to Erase and Format as APFS / GUID Partition Map ( for Big Sur usage ) or for Catalina and below- HFS Journaled and GUID Partition Map.

5 - Once formatted - ALL DATA on the Drive is Lost forever.

6 - Now, Start the Time Machine Backup Application and it should offer to use the newly formatted drive for that purpose - Time Machine Backup

Format for only windows as fat32 or exFat with the master boot records. For both apple and windows exFat

Am sorry but the original quest was to Delete a Backup Drive to make a new one. There was a supplied method to perform that action.

One asked as to how to format the External Drive to be used on Windows. That too was offered a solution.

Now - the user wrote " Thanks a lot, but I have delete the hard with my Mac through disk utiility and now it is not readable on windows. " - where is this all going ??