Four drives here:-
C: OS and programs - 250Gb
D: Sample Libraries - 500Gb
E: Sonar Projects - 500Gb
L: Seagate External 2TB Backup
The exceptions are my Toontrack EZX drum libraries, which leave no install option other than to install on the C: drive. You can move them and alter the registry but I always could see problems down the line there, even if just maintenance.
Projects are also backed up using Gobbler cloud server now too, so even if fire / flood or burglary took my DAW and backup drive, my projects are safe.
I back up just using Windows back up and restore, to the USB Seagate 2Tb external backup drive.
However, I also have a single folder on my desktop that has subfolders for all my software, libraries and all subsequent updates. This is also backed up once a month and kept in a separate location) I figure that if my DAW totally blew up, this one3 folder would be the enabler to rebuild the entire replacement DAW, as with a 3 drive system, I see back up and restore good only for restoring my current DAW following a repair, or replacement of say, one of the discs.
Performance. well, just fine, I cant compare with using just a single drive as I started out with this rig, but even recording with the buffers set really low to get 3ms latency going in, 10ms complete round trip, I have no issues.