Photoshop 6 introduced a few changes in blending modes. Here´s a tip on how to save those older PSD´s, and continue with the Color Dodge/Burn behaviour you are used to:
Courtesy of Mark Hamburg, verified and posted by Chad Baker:
To create a new Color Dodge/Burn layer that will work like the old ones used to, simply Option (Alt) click the New Layer icon and select Color Dodge/Burn as your mode, then check the box to fill your new layer with a neutral color (black for Color Dodge and Difference, white for Color Burn).
To fix an existing Color Dodge/Burn layer (from one of your 5.5 files for example) that has 100% layer opacity, simply bring up the fill dialog with that layer selected, then fill the layer in 'Behind' mode with black.
To fix an existing Color Dodge/Burn layer that has an opacity of other than 100% it's a little trickier, but not much. The process is the same as above, but you must perform a quick trick first. Select the layer, then go to Layer>Layer Style>Blending Options. What you need to do is change the Advanced blending opacity to whatever your General blending opacity is, then move the general opacity up to 100%. Now all you have to do is what you did before, fill the layer in 'Behind' mode with black.