You should consider a SSD drive for your Samples, I'd set it up like this:
C: OS and Programs
A: Audio; your cwp files and the wav files for each
V: VSTi; Samples (SSD)
Be VERY careful when installing, every installer wants to put everything on the C drive by default.
You have to set your download path for Splat and CbB to keep the installers off the C drive.
You have to set your NI sample install to the Sample drive, AND the download path for the NI updates
needs to be set as well (this is where the DATA drive mentioned below is important).
I'd get a 250GB SSD for the OS (C)
Use one of the 1TB for Audio, one for DATA (store your install files, downloads, backups, etc.)
Then the external will hold a second copy of all "critical" data.
backup, Backup, BACKUP! (I grew up in the glory days of NASA, redundancy, redundancy, redundancy!)
Do it, and you can thank me later...
T