I've downloaded the excellent Battlehorn Castle Enhanced mod, which adds a treasury room among other things. Unfortunately the treasury room, with many display cases and items, was made in the same cell as the entire great hall, instead of being set aside as a separate interior cell with a loading transition. This causes fps problems for me. So what I want to do is edit the mod for personal use, and take this one room with the stuff in it and make it into a separate interior cell from the great hall, so that it loads on its own. The mod itself is a lone .esp file (with the DLC Battlehorn esp as a parent as well as oblivion.esm, if that's relevant)

I am completely new to the construction set. Although I think I've figured out how to make the changes I want inside the CS, I'm having a lot of trouble when I save the file. I open the editor, set the mod.esp file as the active file. I open up the Great Hall cell, which has the treasury room in it. Using the render window, I select all of the objects / room I want to move to another cell, and copy, delete, and paste them into an empty cell. I linked the doors between each cell. When I go to save (with the mod.esp as the active file, so saving over it), the construction editor crashes on exit. When I reopen the esp in the construction editor, I got a long list of errors about missing scripts and references in the whole mod or at least maybe the whole Great Hall cell (not involving objects that are in the room I copied and deleted). Oblivion crashes on start up with the resaved esp active. Just testing some stuff, I discover that even moving the room within the existing cell and saving causes the same list of problems. Am I editing the mod.esp file in some way that breaks the whole mod, or is what I want to do simply not possible?

Thanks for any help!

Well, for starters, I would have thought it would be simpler to duplicate the whole cell, then delete the bits from your two new cells you dont want, rather than messing with the cut and paste option (Which personally I find can be buggy).

Edit: Also I think it might help if rather than deleting things in the original cell, you just disabled them instead.

This sounds like a good suggestion, however I'm still having the same problems when I try to follow it. All I did as a test was set the mod.esp as the active file, open up the Great Hall cell, duplicate the cell and save. The CS crashes when I hit exit. I open up and set the mod.esp as the active file again. Oblivion.esm loads, then when the mod.esp file starts loading, I immediately get the following warnings/ errors: first 4 or so "unable to find script... on owner object..", then a whole lot of "Missing object... for reference...." (the status bar shows these errors for the loading of the other cells in the mod, which I didn't modify at all), then a few "could not find reference... for linked points in pathgrid for cell...", some "SetFormID bashing entry in form ID map at..." and several other warnings before the construction set just crashes. Just to be clear, none of these errors occur if I'm just opening the unaltered mod.esp, but any small change I do to it apparently causes all of this.

I'll mess around with it some more, because I really just must be doing something wrong, but I'm really at a loss as to why even a simple change seems to break the rest of the mods data. What I'm trying to do should be very simple and possible, right?

Your problem is the fact that the esp you're working on has an esp for a master. The CS doesn't like that.
Read the http://cs.elderscrolls.com/constwiki/index.php/De-Isolation_Tutorial.

Looks like you're right. I followed the instructions for esp patching using wrye bash and now no longer encounter the same errors when saving. Hopefully I won't encounter too many more problems.

Quick question though, as per the above suggestion to disable instead of delete objects that I don't want in each cell, how does one do this in the construction set? Is this the "initially disabled" tick box when you double click on an object? If so, can you disable a group selection of multiple objects at once or do you have to do it to each object?

Yes, its the 'initially disabled' checkbox, and I think you would have to do it item by item (Although you can use Wyre Bash to automatically restore and then disable all deleted refs in a mod). The advantage is that anything looking for the ref will be able to find it, even though it wont appear in game, which can solve a lot of issues.

I did not realise you were making a new .esp to modify the official one. If you have no intention to release your modifications, you could just make a copy of the official .esp and modify it directly. If you do intend to release it at some point you will have to play about with de-isolation.