How do I move (not copy) a file/folder from my hard drive to a flash drive?
Hold down the Command key while dragging from the source location to the destination location.

Doing this will perform a move and not a copy.

If you rather prefer to use keyboard shortcuts, simply copy file(s)/folder(s) normally using the keyboard shortcut Command + c and perform a move operation using the keyboard shortcut Command + Option + v at the destination location. Skipping Option will perform a copy-paste.

Here's a quick list of tips for managing files in macOS.