Well I have recorded 16 tracks + MIDI  @ 44.1/24 using :
Tascam us1641 interface 
2004 Toshiba Laptop Pentium 4, 2.4 Mz , 1.2 Gigs RAM 120 Gig 7200 HD , XP 32 bit,
Sonar 8.5 32 bit.  
And never had issues in the dozens of times I've  used it. 
Only difference I see is your trying to record a long session and the 5400 RPM drive. 
I did come to the conclusion that 7200 RPM drives are way more important than computer specs for just audio recording. Also getting rid of DPCLAT issues. 
 
Is there anyway to upgrade your Hard drive without a major hassle? 
Does your Laptop have an esata port? Those are supposed to be the best solution. My wifes Lenovo T420 has the esata port. 
 
And I think there has been threads about problems with Sonar recording long projects. 
I've recorded a few  live concerts and what I did was have 10 or more identical blank projects open and minimized at the bottom where they are still visible as little boxes. Each was named with a Number ( 1-GFSSlive , 2-GFSSlive etc)  All were set in record ready mode and ready to go. It took 5 seconds to open one and be recording a fresh project. Mouse click, R.  The first one would just get left underneath to be saved when I had time later.  I used auto save running so the open ones would be safe from any computer shut downs. So it broke the 2 hour concert into 10 or so manageable chunks with very little action on my behalf. I was the lead guitar player at one performance. 
 
This is a case where once these are done it would be great if Sonar had the ability to apply the same mix to every song like a digital mixer will do. 

surely the i/o interface is not relevant to the hd type?
personally i'd go usb3 as it's more widely supported, thunderbolt's a bit of an apple thing

then i think your consideration is: will there also be a thunderbolt port on my next pc? but you can plop whatever hd you like into an external case using the i/o you prefer :-) so thunderbolt now, so the port doesn't feel unwanted, then in the future you can always transplant the hd into another case if you no longer have thunderbolt :-) pc pwns mac ;-)

it's deliberate mis-type of "owns" :-)

USB3 vs. Thunderbolt for an external HD is completely a moot point.
USB3 can sustain ~550MB/Sec
The HD used will top out at about 1/3 that speed - so you're nowhere close to saturating the USB3 bus.
 
The USB audio interface on the X32 is decent, but if you're trying to use it at small ASIO buffer sizes, it'll crap out.
The low-latency performance is nowhere close to a USB RME or MOTU unit.
To achieve solid/consistent performance, you need to set the buffers resulting in ~10+ms total round-trip latency.
The driver is incredibly flexible, it'll allow you to set incredibly small buffer sizes.   They're just not reliable.  
 
When using a laptop for sustained heavy thru-put tasks, you've got to make sure DPC Latency is low/consistent.
A DPC Latency spike will absolutely cause drop-outs/glitches/etc.  Especially if you've got the X32 set at small ASIO buffer sizes...
 
Also, make sure to connect the X32 to a USB2 port (not USB3).
This is especially important if the USB3 controller isn't integrated into the motherboard's Intel chipset.
All motherboards prior to the Z series had USB3 controllers provided via 3rd-party add-on controllers (not integrated into the motherboard's chipset).  This opens the door to compatibility issues (similar to not using a TI chipset Firewire controller with a Firewire audio interface).