Beaver: It could possibly be that Arsenal III corrupted something somehow, and when you try to save it with DatEdit, it goes whack... Try recreating it using only DatEdit...
What you said about the copy function is a really good idea.... One menu alternative to copy the whole unit and one to copy only the current tab. This would only be of interest in the Units editor, of course. As you also pointed out, it should be possible to copy between two open DatEdits

I found a thing that bugged me about when you resize the window.... The only reason to make it bigger than default would be that you get scrollbars in the "Used By" box, and will get in the "Hint" box when it gets enabled, most likely. Thus, these two should grow when you resize the program and making it larger. In the attachment the Used By-box looks pretty eww in my opinion, due to the scrollbars

Secondly, about the Unit size-preview function... What's the largest number these values can take? 128? If it can be 128x128 max, then there should be space enough to add the Preview control to the window in the empty space in the Movement / Sound Tab, as I showed in my previous post...
Speaking of that... Is there correct checking to make sure that values that aren't allowed can't be entered anywhere? For example, values above 255 aren't allowed on the "Supply Provided" value, a Sight Rage above 11 crashes the game according to KA's Units.dat specs, etc...
Also, extremely minor, but IMO the Supply Provided and Supply Produced should be under each other, not next to each other. The same goes for Space Requierd and Space Provided. Ain't I annoying?

[edit=1]When I shut DatEdit down, after messing with most of what I could think of, not saving anything, I got the error "The exception unknown software exception (0x0eedfade) occurred in the application at location 0x7c59bc3f". That's not a nice thing. This error note showed up twice, followed by "Abnormal program termination". I can't really recreate this since I have no idea of everything I did...[/edit]