On Microsoft's "Create Windows 10 Installation Media" page, it says that in order to create the installation media I need:
A blank USB flash drive with at least 8GB of space or blank DVD (and DVD burner) if you want to create media. We recommend using a blank USB or blank DVD, because any content on it will be deleted.
Would an 8GB MicroSD Card in a USB Reader work the same way? (Would Windows even "know" it wasn't a flash drive?)
I'm also a little unclear as to why an external hard drive can't be used for this purpose, but the only options in the instructions are USB flash drive or DVD-Rom (and nowadays it's easier to find a DVD drive at a garbage dump than at a store)...