I literally copy pasted bits of the normal map (without touching the layers), so assumed the RGB layers would remain unafected. I'm still not entirely sure what I did wrong.
Hmm... If you open a dxt5nm file and re-save it to dxt5nm again, what happens to the channels?
As far as I know it, the dxt5nm file is basically just a dxt5 file where red channel information is transferred to alpha channel, and red and blue channels are either black or the green channel is pasted to both red and blue (so the colour channels are the same, and alpha channel different). Now when you have this kind of situation and re-save it to dxt5nm, the saving mechanism again moves the red channel to alpha, which is NOT what you want to have there.
Or, as follows:
Normalmap's RGBA converts to DXT5nm GGGR (first channel is moved to last, and second is copied to first three)
When re-saving an already channel-managed DXT5nm's (GGGR), it converts to frakked up DXT5nm GGGG where each channel now has the originally green channel information.
You can paste stuff but if the channels are already properly managed, save as standard dxt5 instead of the normalmap variant. That should solve that issue.

This is, incidentally, one of the reasons I prefer to manage the channel swapping manually. It prevents mix-ups like this quite effectively when you know all the time where you're going at.
Hmmm...someone mentioned another tool for manking normal maps beside Crazy Bump?
I can vouch for GIMP's normalmap plugin, it's pretty good one, with a good range of settings (depth, filtering etc. etc.). Photoshop's normalmap plugin most likely offers at least the same functionality but I'm not a photoshopper so you'd need to ask about that from someone else.
The main difference with CB and normalmap plugins is that you will need to make several layers of the height map depending on how much different strengths of normal details you want on the map; for example you wouldn't want scratches or other surface detail to be quite as strongly normalmapped as seamlines and actual geometry details like machinery and stuff. But in the end of the day I find it better and quite natural to use. CrazyBump offers some kind of tools to adjust differently sized details of the height map to be mapped with different strengths on the normalmap, but I only ever used the free demo and it felt cumbersome and not quite as accurate as you get with manual processing from the start.