I am unable to move iMovie library folder to my external HD - my passport

You didn't describe what steps you were taking to move the library, and what happened when you tried.

However, to store an iMovie library on an external drive the external drive must be formatted Mac OS Extended (Journaled) or APFS. (Reformatting a drive erases all data on it, so first back up any data that you want to keep.) The drive must also have sufficient free space to contain the library. Do not store an iMovie library on a drive that is being used, or has ever been used, as a Time Machine backup. Also, storing iMovie libraries on flash thumb drives or networks is not recommended by Apple.

To move an iMovie library to an external drive the safest procedure is to go to your Movies folder in the Finder (Go/Movies) and select the library's icon. Then do an Edit/Copy and then Edit/Paste the library into the external drive. Once you have confirmed that the library is working properly on the external drive, you can delete the library from the original drive if you wish.

You can also copy-drag (drag while holding down the Option key) the library to the external drive.

It is good practice to have double backups of your iMovie library on separate drives in case one drive fails. A Time Machine backup is very handy, as it will backup your library periodically, so that you can restore it to an earlier point in time in case of a failure of the external drive or corruption of the library.

Finally, before moving your library, select its name in the iMovie sidebar and do a File/Consolidate Library Media to insure that all media used in the projects is carried over. See this link for more info:

https://help.apple.com/imovie/mac/10.1/#/mov882dee351

-- Rich