Oops, correction to my previous posting. I remembered that the iso download I'd used successfully before in VirtualBox wasn't actually the same download I was using with USB Startup Disk Creator. So I just did an md5 check -- sure enough, the version I'd been having all the issues with was corrupt. Downloading another version (and verifying its checksum this time!) solved all my problems.
Oops, correction to my previous posting. I remembered that the iso download I'd used successfully before in VirtualBox wasn't actually the same download I was using with USB Startup Disk Creator. So I just did an md5 check -- sure enough, the version I'd been having all the issues with was corrupt. Downloading another version (and verifying its checksum this time!) solved all my problems.