These issues have been discussed and if you look for phrases like in use by another program of file is locked in the forum search, a ton of possible causes and suggestions on fixing them will come up. in short: You have another process interfering with your file I/O which could be a virus scanner, an indexing service or network stuff. Depends. also, since you seem to use linked EPS files, this could be caused by actually only one file in teh document being locked, but the main document not being affected. Hard to tell, so provide some more info.
Thanks for your reply.
Is there a way to search only Illustrator forums? For example, when I search "in use by another program" I get entries from all Adobe programs, many pages, and haven't found one related to AI CS4 yet.
I am not locking any files, and the linked images aren't locked. The issue happens also with only vector art -- no linked images.
I am on Mac, OSX 10.5.8. Illustrator CS4. Don't have indexing service, virus scanner, and am working on a local drive.
I'll keep combing the Forums, but wanted to give you the additional info.
Sorry, I discovered how to narrow the search to Illustratory.
Does it happen if you use AI files instead of EPS?
When you have entered the search term (use " and " to get only the words together) and see all the irrelevant threads, click More Options and scroll down to Illustrator in the Where dropdown (just Illustrator); you cannot do it from the beginning; you may also add CS4 in the search term, outside the " ".
Edit: too slow in writing.
"Save" Problem not solved yet -- just the "Search Forums" issue.
Will try using .ai files per Larry's suggestion.