Hi,
There could be a lot of reasons: from permission problems to corrupted drawing database.
You wrote, that you have "got a new iMac". What OS installed on your Mac?
Have you tried to save the drawing to another location (you can try to save it to USB drive)?
Have you tried to AUDIT drawing files? -EXPORTTOAUTOCAD, -WBLOCK entire drawing?
Could you post one of those files here (or send me by e-mail mkanaev@yandex.ru )?
Maxim