After Time Machine erase, Preferences shows entire HD free but mostly used, so which is it?

I got a new Mac, so the old Mac provided a good backup. To clean out the old external HD Time Machine, I erased it to start with a new Time Machine backup. After erasing, I get the message below showing the disk capacity bar showing all black (i.e., not blue as being used), but indicating 663.7 MB as used. Then to the right of the bar, 999.16 MB is indicated as free. So which is it: Do I have all or 1/3 free for use? Did the erase operation work or fail? What is 221.4 indicated as purgeable? If to get a clean Time Machine HD I need to purge the 221.4, how is this done?