Files & Folders require Admin password to change

I am using OSX 10.12.4 on an iMac with external USB drives.

I purchased and downloaded a very large photography course onto an external hard drive.

It came as a 30.6 Gb zip file and decompressed into files in nested folders.

If I try to move any of the files or folders they will copy not move and it asks for the Admin password.

If I rename any of the files it asks for the Admin password.

They are not Locked or Stationery Pad.

I looked at the Permissions for the files and folders and they were set Read Only for me, staff, everyone.

Changing the Permissions to Read and Write does not however change that they keep asking for the Admin password whenever I change or move them.

It is not the drive, because other files and folders and anything I create myself on that drive do not ask for the Admin password.

Where is the setting to turn this off and can I change it for the whole Folders and files at once?

Peter