Insider update stuck

You can keep waiting (I wouldn't, but patience has never been one of my more outstanding virtues), or you can forcibly reboot your machine. Then, you can either install and try the Windows Update MiniTool (which often works when Windows Update does not) or you could visit the Microsoft Catalog and manually download KB3206309 (except I can't find it there -- does anybody know of another source for this update? I'm thinking it's fast ring only, and doesn't hit the catalog until the update goes out to the slow ring. Is that correct?).
I guess that means you should try WUMT (Windows Update MiniTool) and hope for the best!
HTH,
--Ed--

PS: here's some interesting discussion on the update from Martin Brinkmann at Ghacks.net: Windows 10: Cumulative updates KB3213522 and KB3206309 - gHacks Tech News

Glad you got it sorted. I wonder what it is about attached media that sometimes causes issues? Same thing happened last year on my Surface Pro 3: I had to remove the SDXC card from the machine to get it to upgrade from Win 8.1 to Win10. Strange!
--Ed--