The thing you have to watch out for with external hard drives these days is that many of them can't guarantee a spin speed. For optimum audio use you need to be looking at 7200rpm speeds, but few of the big name externals (WD, Seagate etc) advertise the spin speeds on their products any more. They used to. Steer clear of any "green" models that have variable spin speeds to save energy.
Take a look at this old link to a discontinued Western Digital drive, and note the stated spin speed of 7200rpm:
http://www.bhphotovideo.com/c/product/522052-REG/Western_Digital_WDH1U10000N_1TB_My_Book_Essential.html Now let's look at an equivalent model as is offered now:
http://www.bhphotovideo.com/c/product/841444-REG/Western_Digital_WDBBEP0010BBK_NESN_1TB_My_Passport_USB.html No mention of spin speed at all.
It's worth shelling out more for the hard drive if you can, models like Glyph and LaCie offer spin speeds of 7200rpm and are thus more suitable for audio. Some people will tell you to steer clear of LaCie, but I have no experience of them.
But if you get a decent external HD with a good spin speed, you'll have no problems at all and you won't lose any performance over using your internal drive. In fact for things like recording audio and streaming samples, you'll see better performance than you would if you were using your laptop's internal system drive. Sonar will have no trouble at all finding the VST's, you just specify the path to the external drive in the VST settings.