> Is there some way to determine that the mouse button is being held
> down, not simply a single MouseDown Event?

Sure: return True from MouseDown, and now you’ll get repeated MouseDrag calls as long as the mouse is down. (After which, you’ll get a MouseUp event.)

> I would like to repeat code constantly in the MouseDown Event as long as it is being held down.

Simple move it into MouseDrag, and return True in MouseDown.

REALbasic does not expose a Lock/Unlock construct for file access. What’s more, when you open a file in REALbasic, you have no control over the share mode of the file (unless you open the file yourself using OS APIs). RB only opens files for read sharing on Windows, not write or delete sharing.