I usually refrain reccomending registry-changes but if it only works this way, so be it.
But do me a favor and change it back once you´ve installed it on your other drive.
1. Deinstall the whole launcher + client, also delete the folder(s) under C: and go to "Documents" to delete "BnS" Folder. Be sure that nothing is left, otherwise it might cause problems.
2. Open "Run" (windows-button + R)
3. Type in "regedit"
4. Go to: HKEY_LOCAL_MACHINE\Software\Microsoft\Windows\CurrentVersion
5. On the right side you´ll now see different keys. Go to "ProgramFilesDir" & "CommonFilesDir" (x86)
6. Double click on both of them and change the written:
C:\Program Files (x86)\Common Files // C:\Program Files (x86) to the correspondig drive you have in mind D:\Program Files (x86)\Common Files // D:\Program Files (x86) - for example)
7. Now the default should be set to the drive you´ve chosen. Restart your PC now and proceed with step 8.
8. Run the launcher file and look out for options to get the Cheap BNS Gold. Even if now D: // E: or still C: is listed, try to type it in manually.
9. Let it download & install the client now. Occassionally check if your chosen drive becomes fuller or if a "NCSoft" folder has been now created under D: // E:
10. Don´t forget to revert your registry back once you´ve installed it successfully on the other drive