Because the files being manipulated are typically to large to fit into the computers memory, data must be copied from the original file into a new file (complete with any modifications you have made). This means that you need enough free disk space to store the modified file and the original file.

It also means that the file may take some time to write, as copying TB's of data is slow!

Because the Large File Editor builds the working document from the original document with changes 'overlayed' onto it, when you save over the original document, it effectively has to re-open the document, this causes the undo buffer to be .lost. The undo buffer is NOT lost if you do a Save As operation, however a file lock will still be held on the orginal file.