regretfully, i have to take back what I said about seperate being cheaper than a portable HDD
Portable HDDs are cheap ass these days.
BUT
If you want a decent portable
SSD, then the seperate option is definitely still viable.
Also you should consider your interface.
I use eSATA, which is legacy to most people. However, in terms of performance and consistency I get a lot more bite with eSATA than USB 3.
I say this because my W510 is on dual boot, and the 2nd OS must come off an external drive.
Having tried Windows 10 over USB 3 and eSATA, the latter clearly destroyed the former in performance. Even though both transfer rates are theoretically the same.