It looks, like nobody here uses CS4, Brian.
My CS6 files are saved correctly. Are you exporting 16bit or 8bit PSD? Have you tried both versions?
If I change the name and/or format type, will it go back as a version in Apeture?
PhotoShop should save in exactly that format, that Aperture is sending, and for PSD files the filename extension should be ".psd" in lower case letters, for TIFF files ".tiff". If Photoshop should be changein the extensions when using "Save", try "Save as" and make sure the extension is correct.
Could you use "tiff" format, or do you need the layers?
That's what I get for holding onto old software. In any case, I've tried everything I can think of: PSD 8 and 16-bit, TIFF 8 and 16-bit, JPEG.
The weird thing is sometimes everything works fine, than the next time Aperture gives the error message.
Instead of using the Photos > Edits with Photoshop CS4 ... I may just work around it with the RAW > Export (Version) As> PSD to the desktop, open in CS4 and then Save As back to the desktop, and then drag it back into the Aperture project. It's cumbersome but it works.
Thanks for responding,
If the problem is erratic, try to ensure, that not your Aperture library is to blame.
If you can edit without problems from a new Aperture library, just created for testing, then I'd run the Aperture library first aid tools - repairing permissions, repairing the database.
Another test would be, if CS4 is working well as an external editor, if you are testing using a different user acoount - bare bones - with only the default settings.
One problem - according to Adobe's site, CS 4 is not supported for the recents MacOS X versions, so you may have a problem with the sandboxing in Mt. Lion. http://helpx.adobe.com/x-productkb/policy-pricing/system-requirements-photoshop. html