Which system folders are safe to junction off to an external HD?

I want to reroute the folders that are not required at boot and are accessed only on explicit user request, such as SoftwareDistribution (the updates are set to manual). Example:
1) Move SoftwareDistribution to external HD.
2) Execute in C:\Windows: mklink /J SoftwareDistribution "G:\Backpack\Windows\SoftwareDistribution".

Which other folders are substantially big and safe to move this way? Thank you.

Take a look at the relevant link in Wikipedia. While it's possible to redirect most folders to other places, some are forbidden and a few others can have side-effects if done. SoftwareDistribution doesn't seems to be one of those.

I was hoping for a rough list to save some time, but I appreciate the answer anyway. I guess I'll have to poke around a bit. I have a backup solution, so no worries.