I saw no advantage to installing on an external drive sorry.
However, on the t5740 which has 3GB Ram and only 2 GB flash (and it's network chip is not supported by Snakeoil) I tried installing another Ubuntu distro.
Initially had the same experience you are as it would not boot from the attached drive (even though it was sata connected internally !)
There were two workarounds.....
1) I could disable the internal flash storage in Bios.
2) I could place the boot partition on the internal flash storage and the / and home partitions on the attached hard drive when setting up with the Ubuntu installer.
Either worked but note that the t5740 uses conventional bios and the t520 has UEFI so may not be as easy to configure.
On the t520 I can boot from usb drives but had to enable the legacy options in the t520 bios....let me know if you need help with that.... I should be home this weekend and could look at the settings.
I'm a believer in multiple redundancies so when I spot a bargain I general double buy.... so I have 2 x t520 and quite happily (after researching and evaluating as much as possible first) experiment. YMMV

My advice would be that even your 8GB of internal flash storage is enough to run Snakeoil from and, unless you have some other requirements or enjoy the under the hood stuff ?, then enjoy the music...

Cheers,
Bob.