BcdEdit: "The requested system device cannot be found"

Today I was wrestling a baffling error message displayed by the Windows bcdedit tool:

The store import operation has failed.
The requested system device cannot be found.

I had been trying to create a new BCD store and add a new OS installation to it. This same procedure had worked many times before in virtual machines, so I was very confused as to why it was failing here. (The hard drive did not become bootable; it didn't even show the Windows bootloader OS selection screen.)

It seems the problem was with my memory. Though I had created the partition and ImageX'ed the OS onto it, I had forgotten to mark the partition as active in DiskPart. Typing "active" at the DiskPart prompt marks the currently selected partition as active, making it eligible for booting. The disk then becomes bootable, as it has an active partition.

Thanks to this SuperUser question for reminding me that active is a thing. (Its answer also mentions a rare issue involving USB3 boot flash drives and Windows PE.)