It might work if you take ownership of the external HDD folder, right-click the folder, select Properties > Security Tab > Advanced at the bottom > Owner Tab > Edit > Highlight your username and put a tick in ‘Replace owner on sub containers…’ and Apply > OK. Now go back to the Properties > Security Tab, highlight your name and give yourself full permission (tick Full Control), if necessary. Have a look here click here
In future it may help you to add ‘ownership’ to the right-click menu.
Two registry changes will add or remove ‘Take Ownership’ to the right-click menu on most folders/files. Download them both here click here
To use it, right-click a folder or file > Take Ownership > Click Continue for the UAC prompt. When it’s done, a command window will briefly pop-up.