You know, this is the reason Bryce needs displacement. With displacement even a scan of bread would be fully 3d. I agree with what others have said, volumetrics do not like complex geometry, not that a slice of bread is complex, but volumetrics are very sensitive. I learned this when I tried to model a tornado and then add volumetric mats to it for a storm scene, the funnel model was too complex for the volumetrics.

If I was going to use the scanned image, I would use it with very little if any bump, the shading of the bread itself will probably be the best you can do.

I would say that voluemtrics are well suited for bread so when you get the files reinstalled it will likely be fine. I also think the problem is your install of Byyce, not just the voluemtric folder. you might need to reinstall Bryce. Just make sure yousave all of your user presets for objects, materials, skies, and DTE textures to a safe place so the new isntall does not copy over those files with blank versions. I have lost alot of info during reinstalls for this reason.