I won't/can't speak to the finer technical points, but what I did, about a year now, was purchase a USB 3.0 compatible 7200 rpm external drive and a Cool Max enclosure - from NewEgg I believe.
 
The advice here about leaving Sonar installed on my laptop's internal drive and using the external drive for project files and audio is exactly what I did.  Works great.

ricoskyl
So solid state is the smart of it's combined with a generic HD. At that point, it seems the HD specs would be a lot less important. Interesting.

Sorry, I don't check this forum everyday.  A lot of the stuff your talking about is now obsolete.  Raid, NTFS, FAT. Honestly just buy a smallish SSD drive whatever you need, perhaps buy a second storage drive.  They're so cheap now, we're in a different time now.  Gosh most of the advice in this post is last decade, and I should know I've been in this caper now since 1999.  All hard drives these days' are NTFS, FAT 32 was the old standard and meant that you couldn't haven't have files any bigger than 4gig.
 
I run massive video files off my hard drive and I'm only just reaching the limit.  And by massive video files, we're talking 50fps and 60fps over laid on top of one another...so 6gig and more going through the pipe at one time.  Music wise, I was pushing 48 tracks of audio, effects and 10 VSTi's last year and only at the very end did I need to increase the latency.  All through the recording I was able to use Guitar Rig 4, whilst recording-and not have any audible latency until the very end.
 
A swapable HD wont effect Sonar, I've run a swapable HD system since 2010.  If you run into any problems just clear the INI file.  Don't ask me off the top of my head about the details.
 
Just run Sonar off the laptop drive, along with samples and the like and everything will be fine.
 
Cheers Ben

Ben is right, IF you want a 128GB drive. there is no denying that SSD is way faster, as they have no moving parts. If you want higher storage capacity, you may find SSD to be a little cost prohibitive, in which a higher capacity HDD may be a little more forgiving on the wallet. If you decide the latter, the 7200 rpm USB3 advice still stands.

SSD prices have come way down.  Yes spindles are way cheaper, but now USB 3.0 SSD is the way to go, and  I even use a USB 3.0 thumb drive that's 128G for recording sessions that I got for around $50.  Makes it so easy with portability and ease of use, and the write speeds are close to SSD.
 
If you have to buy a mechanical hard drive, I'd recommend one for backing up projects to, but use SSD or flash storage for writing I/O.