Same issue here, whether its via external hard drive or internal ssd, i have both pc and imac pro / both of which encounter the error - my pc more than my imac however.

Same problem here in Windows 10, in July 2021.
This only happens when saving an AI file.
The only solution that worked for me is to reset Illustrator Perferences: Open up Illustrator holding CTRL+SHIFT+ALT.

Once error message shows no file will save even a simple square. This has been an issue that comes and goes.
Often its the first file of the day from a fresh start up.
One or multiple reboots clear it up the issue. The loss of time and the work performed is compiling into a real issue for us!

Unknow error when saving .ai in version 25.2.2

My operating system is 64-bit and my Illustrator is v25.1. Today my desktop crashed while working on Illustrator. Had to restart it, which forcibly shut down Illustrator. Once it restarted I opened Illustrator but there was no internet connection so it didnt activate the font used in the work and so had to delete the "restored" document. Once I connected the internet my account was signed in and could activate the font. But now when I try to save the project, "An unknown error has occurred" appears.

I can't save anything in any way, tried everything! The "an unknown error has occurred" message keeps coming. What to do?

The file won't Save, Save As or Save as a Copy. I opened an existing file and created new artwork. When I copy the new artwork, to a new file and try to save, the same message comes up "An unknown error has occurred."

Something common we've found is this happens on files with Myriad Pro. Our workaround has been changing to a different font so we can save the file, then clean the font cache and change the font back.

just updated to Big Sur this weekend on Mac. Was able to save my file periodically then was not able to.
Did open the file from a server that was still in the process of syncing. Also use Font Suitcase.
tried renaming, saving as different file types and locations (and back saving it). Nothing worked- lost 20 mins of work. Blank document with just square would not save either.

1)
Illustrator 25.1
Mac Os 10.15.7
2)
Saving a document which I saved successfully just 10 minutes before
No hidden/blocked layers or objects (sometimes it's the simplest vector file with only 1 layer)
Occurrs with local disks and external hard drives
3)
My expected result: being able to save my work!
4)
Not knowing when the unknown error will attack, every save is a gamble. When the error occurs I can only take everything to photoshop as an intelligent object, close and open illustrator again, open the intelligent object and hope that now it will save. sometimes it does, sometimes it does not. There is no logic to it.

an unknown error has occurred illustrator

After updating to Illustrator 2021, I get a "can't save illustration" error when trying to save as eps

When trying to save changes to a previously created file, I get an error that just states "An unknown error has occurred".
Trying to resave as a new file results in the same error.
Unchecking "create PDF compatible file" lets the file save but that is not a solution, these files must be PDF compatible with editable text and artwork.

Removing presets as instructed in another troubleshooting document does not solve the issue, removed presets still exist in the program even when removed and they cannot be deleted from the Illustrator dialog box.

Wiped and reinstalling Illustrator does not solve the issue.

I have a Mac, running OS 10.13.6, Illustrator 24.3

I want a solution to this problem

It can't be saved even just create a new file with 1 rectangle. also a blank new file can't be save.

Files will not save. WTF IS WRONG WITH YOU ADOBE WHY YOU GO TO ****

Mac 10.15.6
This is now the third time this has happened to me. All of sudden Illustrator loses the ability to save files. When I try to save a file - a window will popup and say "An unknown error has occerred" and nothing I do will fix it. I can save the file as EPS or SVG - but no PDF or AI

Even Opening a blank doc. and saving with nothing in it results in An Unknown Error.

I just ran into the same problem on my PC. Using Suitcase. Even when font outlined... File will not save.

This isn't just a MAC issue, I have PC and just ran into this same issue. Thanks to your post, I was able to outline fonts and save just fine. I'm using Typekit.