same issue, never had this show up before. macOS high sierra 10.13.2, iMac 27" , 32GB, NVIDIA GeForce GTX 780M 4 GB. Files are inside drop box folder, but that was never an issue, used to work perfectly.

Well, I take back saying that turning off the icon preview worked. I started getting the error message again yesterday with it turned off, so that isn't the solution.

Same issue. Here's my environment:
Started happening on my MacBook Pro (early 2015 model) after upgrade to macOS High Sierra.
I save to a Dropbox folder, and have since I started working on this computer. No issues since then

I also have a MacBook (mini Mac) that I noticed the issue on a few months back -- running macOS High Sierra (upgraded there earlier than on the MacBook Pro). Also saving to (the same) Dropbox folder system.

As such, I'm assuming its something with the OS.

Running CC on both computers, and Applications update regularly.

Does not happen in other Adobe products, only Illustrator.

I've tried:
- Turning off "handoff"
- Turning off "icon preview" (per suggestion below)

I'm open to being contacted for further testing!

I have had this problem since upgrading to High Sierra, too. Someone in this thread suggested turning off "icon preview" in the Finder (View>Show View Options). This seems to have worked for me, BUT it is super annoying to me to not have the previews for files I've saved to my desktop. Please figure out how to fix this so the icon preview can be turned back on.

Hello. I JUST upgraded from OS X Sierra to HighSierra last night (Jan 21). This afternoon (Jan 22), I'm using Illustrator for the first time after the upgrade. No problems at all until I hit Save and immediately get the message here reported (one I've never seen before today). I am saving to a USB external drive, just like I've done hundreds of times before without any odd error messages. The message pops up only after a couple of saves. If I restart Illustrator the message does goes away but for a while only.

@ashutosh, I am getting this error using and I am using a Dropbox folder for file storage. A Dropbox conflict certainly makes sense. I'm using AI 22.0.1 Mac. It was also happening before I upgraded to CC2018.

Same. Saving to desktop. Illustrator CS6 (v16)

I have the same problem saving files to my desktop: each time I receive the same message. It's very annoying and wastes a lot of my time.

Same here. Upgraded to High Sierra OS. Only saving to desktop. Still get error.

Upgraded to MacOS 10.13.2, then signed in to a new departmental dropbox account, then started seeing this message. I don't use the dropbox account in my work flow; .ai files are opened from and saved to the HD.

Issue started this morning once I upgraded to High Sierra Version 10.13.2. I am only saving Illustrator files to my desktop. Working on an iMac (Retina 5K, 27-inch, 2017)

I am having this issue as well. Not saving to dropbox or any cloud service - just my desktop. I am working on MacOS High Sierra Version 10.13.2 working on a Mac Mini

I have been experiencing the same issue and yes I do use dropbox, but I have been using dropbox for months now and this was never an issue on previous versions of Illustrator.

me too. only in illustrator. no wierd plugins. illustrator 22.0.1. mac os high sierra 10.13.2

Same here, and it's driving me crazy. Photoshop is doing the same thing, except it comes up as 'Are you sure you want to replace...' dialog box. No networking. No Dropbox backup. OSX 10.13.2, AI 22.0.1

ADDITIONAL INFO: I am using various Astute plugins including Auto Save. If this might be connected.

Same issue. Files on HD. Illustrator 22.0.1 and High Sierra 10.13.2.

I'm frequently seeing this problem too.

Illustrator CC2018
High Sierra
Saving to my external RAID drive

Yes, I understand Adobe has never supported network drives. I got that years ago.

The "modified outside Illustrator" error is seen on external drives as well as my machines internal drive. MacBook Pro with Touch Bar 15-inch.

Nothing to do with DropBox or Google Drive. No networking involved here.

Except for Adobe's licensing servers and Typekit.

And to add to the last comment I made...With those files I have saved to Dropbox...I get the popup when I am saving even to my hard drive and not to Dropbox. Usually I save my files to hard drive AND then upload to Dropbox later from my hard drive. So I get the popup even when I'm saving only to my hard drive and not to Dropbox.