How to edit using Photoshop in RAW

Question

I have setup Photoshop CS4 as external editor. However, Aperture export to Photoshop in TIFF format... Can I set it to export as RAW format ??

Answer 1

You're missing the point. Aperture is, among other things, a RAW converter. It performs the same role as Adobe Camera RAW (ACR). You use one or the other.
If you want to go through ACR, then export the master from Aperture, run it through ACR, edit it in Photoshop, and save the result in Aperture for asset-management purposes.
"It can't be so difficult!" misses the question. In fact it is difficult because Photoshop does not handle RAW files. ACR does that work, and hands off a PSD or TIFF file to Photoshop. So what goes into "Photoshop" as a RAW file is actually going to ACR, and then being converted to a different format. This file conversion is difficult if not impossible for Aperture to keep track of -- it's creating a new file.
BTW, Lightroom works the same way w.r.t. ACR. They're separate RAW converters that use the same engine underneath.