I just got a new iMac, loaded the Autocad LT 2013 trial version. I can then open & work in my DWG files (which had been saved on my old PC in my old version of ACAD LT), but I can't save the file. I've tried to SAVE AS to a new filename, but I always get the message I'm attaching to this post.
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)?
Could you post one of those files here (or send me by e-mail firstname.lastname@example.org )?
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