I've finished migrating data from several drives on to one newer disk. The older drives were a mix of old mechanical drives, thumb (flash) drives, and SSDs.
Now I will dispose of the worthless drives and sell the rest. In order to sell the drives, I should be completely sure that the data cannot be recovered (e.g. by a scammer, blackmailer, or pryer).
On each drive, I have deleted all the files.
What else needs to be done?