So I got this message:
MSP Error: 28017 SQL Server Setup cannot write to removable media. To continue, specify a directory on a hard disk attached to the computer where SQL Server will be installed.
And I'm installing to locally attached storage. After about 20 minutes of cursing, rebooting, and cursing some more, I went to My Computer and disconnected all my network drives. My logon script helpfully maps network drives, I'd forgotten about them since mapping drives sucks and I don't ever use them (UNC paths for life!). So I unmapped them and suddenly it worked.
Stupid bug, bad error message, and stupid logon script.