I recently did a fresh install of macOS Catalina and was struggling to get Sonarr to move files, that were downloaded by Sabnzbd on my hard drive, to an external USB drive. I trawled through this forum and the internet more generally, tried all the various suggestions, and nothing worked.
By coincidence, I tried to delete some files off my external hard drive, and every time I tried to do so I was prompted to provide an administrator password. It became a pain, so I searched this issue on the internet and came up with the following suggestion, that worked:
Open the Terminal app—it’s in your Mac’s Utilities folder. Drag the Terminal window so that you can see both it and that hard disk drive’s icon at the same time.
At the command prompt type
sudo diskutil disableOwnership
…followed by a single space character. Drag the hard disk drive’s icon to that Terminal window, and then then press Return . You will be prompted for an Admin password. Type it (it will not be echoed, not even with •••• characters) and then press Return.
Having done that, I then tried Sonarr and, voila, Sonarr was able to move files from my hard drive to my external drive.
For those struggling with this issue, try the above, it certainly worked for me!