I've never stored content on my C drive, and have also never had any problems with finding it after doing a custom install and selecting the paths during setup. If the user, and iClone, have full rights to access the content drive - and you can view the contents of all content folders using Windows Explorer - then you could take a look in the Windows Registry to confirm that the paths have been correctly. Run Regedit and check that the file paths are correct under:
Computer\HKEY_LOCAL_MACHINE\SOFTWARE\Reallusion\iClone\6.0
If the file paths are not as you expect them to be then the safest way to correct the problem is to uninstall iClone, run a Registry cleaner and do a fresh install using the content paths of your choice. An uninstall/reinstall does not normally remove any custom content that you may have created, or purchased, but a backup of the folders is always the best way to insure that nothing gets lost.
Gerry
System: Win 10 Pro (21H2), Asus X99-E WS, CPU i7-5930K -3,5 GHz, 32 GB DDR4 2666-16 RAM, NVidia GTX 1080 Ti GPU - 11 GB VRAM(Driver Studio-536.99), Samsung 850 Pro 512 GB SSD, 6 TB HD storage.