I have seen this message even when saving files to my desktop. The files seems to start out okay and I don't see this message until I have saved the file many times.

I have NOT been saving to Dropbox and still, I get this message. For the most part I save either to an external USB SSD or to my desktop while working on files. My desktop is synced to iCloud however. I am running latest version of Illustrator.

So I got this "problem" when I want to save my illustrator file. Sometimes when I click save, it tells me that this file was edited outside of illustrator and asks if I want to continue. I can click Yes and everything is fine, but getting that message is kind of annoying and doesn't make any sence, since that file is only on my desktop and nobody else can even take a look at it or edit it from somewhere. Sometimes I just change a color and that message appears when I want to save that file, sometimes there is no message...
I have Illustrator CC by the way (a german version if that matters).

Not just DropBox but it definitely seems to be related to saving to a "non-local" folder. I see it when I am saving to either a DropBox folder and also a GoogleDrive folder.

Holy cow, I just upgraded to High Sierra and immediately discovered the same problem as everyone else with Illustrator (CC 21.1.0) and Dropbox(v39.4.49). I save to a folder on my hard drive, which is synced to Dropbox. After that file has been synced to Dropbox, something mysterious happens and Illustrator thinks the file has been altered (name change, or something?) outside of the application. So, when trying to save that file again from within the application, the message pops up. Only happens with Illustrator, and started happening immediately after upgrading to High Sierra (10.13.1) Been working in this same manner for a couple years with no issues until now.

I don't use dropbox but sometimes i get same message since i have updated High Sierra (not while working on ALL of my illustrator files - i don't know what causes this problem).

Running Illustrator CC 2017.

Hi, I don't use dropbox but I do use iCloud and also I use an external server (lord knows where it is) as I work at home. The problem occurs when I use the the drawing tools and then try to save to my desktop which is connected to iCloud. It doesn't happen when I update a drawing using the data plotting devices. I am running OS 10.13.1 on an 2015 iMac and also a 2016 MacBook Pro also running OS 10.13.1. Running Adobe Illustrator 2018 version. Thanks

Same problem, happens with high sierra and illustrator 2018. Happens only with Illustrator.

Same Here! Really Anoying.

I have the same problem, just how Nick Rougeux explained, so frustrating!

same problem!!

I get the same thing. Only started after the OS update (10.13.1) and CC18 (22.0.1).

Same issue here - after updating to illustrator 2018 and Mac OS 10.13.1 when saving .ai file I get the message 'this file has been modified outside of illustrator...' very annoying. No other CS apps are open. Please make it stop.

i am saving onto my mac hard disk. it's an ai file that i have reopened and made changes to. Using CC 2017, Illustrator 22.0.1. Getting the exact same message. This didn't happen before updating to High Sierra 10.13.1 on mac. Make it stop please.

Same problem here when saving as .ai

Problem seems to occur when saving a document for the second time and onwards.

I can also confirm this issue. Experiencing the exact same issues as others have mentioned on this thread with AI CC after upgrading the mac to macOS 10.13.1.

I'm on CS6 standalone, Ai 16.04, and started getting this (not every save) after I upgraded to the latest MAC OS 10.13.1, and updated Java to go with it (had to or AI would not open). I don't think it's related to AI, I think it belongs to the OS

Saving an AI file on desktop, not networked, no other program running and not sharing the file but get the 'this file has been modified outside illustrator' message.

Same issue here, work is saved in Dropbox.

Yep same problem here as well. Saving an ai to a folder in documents. Started happening after updating to Illustrator CC 2018 22.0.1 and macOS High Sierra 10.13.1.

Yep I'm getting the same since upgrading to High Sierra and CC 2018. All my live work is also on Dropbox.