For the custom content, this is the way.
Custom Content Import CC
First, you need to create a custom content folder (I suggest that you do it on another drive)
Both of mine (Realussion Templates and Reallusion Custom) are on the second drive.
Paths:
D:\Reallusion\Reallusion Custom
D:\Reallusion\Reallusion Templates
Second, you need to define CC and IC to look in those folders/paths.
Custom Content in Multiple Drives
Third, for the downloaded content that comes with the vanilla CC and IC, you need to update metadata.
Don't forget to download that content on the first run, before blocking connection in Hosts or in firewall In-Out rules
You can find the models and metadata here:
REALLUSION Mocap Motions for IC8 and CC5 & 2457 MIXAMO motions & CC IC Metadata
For running and registering metadata follow the instructions in ReadMe.txt
And there are new versions of CC and IC - CC5 and IC8 here:
REALLUSION iClone 8.65 & Character Creator 5.05 & Plugins