How do I add permissions via command line for "everyone" on external HDD

Use takeown to take ownership of the file

takeown /r /d y /f * 

^ Recursively takes ownership of all files without prompting "are you sure".

Follow it up with icacls set the access control list

icacls * /t  /grant Everyone:F

This will recursively grant Full access to user group "Everyone" to all files in the folder.