Simply put, this is a memory issue. I wish Adobe would address this, I've emailed, posted on forums etc. I'm not positive as to the REASON this issue pops up,it seems that when opening a file with a lot of PSDs on it, the system runs out of memory rendering.
After dealing with it every day for two years now, I've learned what to do... sorta.
When you have a memory intensive file on screen, say an advertisement with many placed objects, Illustrator uses a lot of RAM to render/illustrate the page and environment, so much so as to not allow the opening of another file or placing a graphic on the page, EVEN if the first file is closed. The only solution seems to be to close Illustrator and start back up.
There are several things you can do to mitigate the problem somewhat.
Make sure you have as much RAM as physically possible available for Illustrator to use. This means close down other applications that might be gobbling RAM. Next make sure your PC/Mac is pumped full of RAM, RAM is fairly inexpensive now days so splurge. In Illustrator you can turn off the full preview, or even use pixel preview to save RAM.
Regardless of these steps, however, you will encounter the error again, fortunately CS4 doesn't seem to screw the session over like CS3 did with no way to save.
Intel CoreDuo2 6600, 2.4 GHz
4 Gig RAM
Windows 7, x64
** I don't think more RAM is really the answer, this problem might just max out whatever RAM you have.
The memory you have install is in fact not the issue it is the use of the memory by Illustrator.
One of Illustrator's Teams biggest focus should be on addressing Illustrators ability to handle large projects this mat or may not be the issue here.
This might or might not help you. One o f the things we use to do when working with linked files which does not seem to be done as much now is to place all link files in the same directory as the document. Not just in the a folder with all the links some where on some other drive etc.
Also optimizing the disk helped but had to be done over if you worked on the document extensively.
Keep in mind Illustrator can only use 3 GB of memory max.
I would discuss other issues that might play into this but as I think Adobe should really d something about the use of Memory by Illustrator I also point out that won't help you as CS 4 is the current version.
Brilliant reply Wade. This makes a lot of sense.
I use Illustrator much like others use InDesign, it's my primary page layout application for newspaper and direct mail ads. I use Illustrator because it is much easier to adjust the graphics on-the-fly rather than the change-and-update workflow of InDesign. Also, there are rarely large swaths of text on these ads, just pricing that is embedded in graphics.
Your note that Illustrator only uses 3 Gigs of RAM explains why once the 3 Gig barrier is hit, Illustrator kicks an "unreadable" error. I'm a bit frustrated that there is no official word on this issue. But perhaps we few who push Illustrator in this way are encountering the error.
Consider this Illustrator is often used with certain types of mechanisms that can probably be handled better for instance effects.
Currently for some reason the default for the Document Raster Effects Setting is 300 ppi excellent for out put but unnecessary for working and can cause the problems you are seeing. Why it is necessary to render the Raster Effects rather than just using a mathematical preview of it is beyond me
and only set the effects at output time.
Clearly would reduce the size of the documents and increase the speed enormously and probably eliminate most if not all of the error messages concerning memory.
I think it is not really that impossible to make a transition from rendering with in the live file as opposed to only rendering on output.
I suggested this many times and even the folk here on the forum do not seem to like the idea that the file would only be represented by a preview with actually not having really become rendered product intact.Even though many of the users here would like to see Illustrator be re engineered?
Reducing the Raster Effects setting might help a little bit 72 ppi use to be the default which speeds matters up.
Someone will stop by to say that raster effects settings has nothing to do with placed images, but I assume that if you have effects in the files that might contribute to the problem.
But in the end they will eventually have to address this problem as their user base is using Illustrator for many things it originally was not intended to be used for but has become such a tool and the demands of the users make it necessary.
Increasing the amount of RAM AI can use will be necessary or the amount of RA it requires to do increasingly complicated tasks.