I'm trying to boot my Mac mini with a USB-connected Mavericks boot disk.
Holding down option when I restart gives me the choice of selecting either my main disk (Macintosh HD) or the boot disk (Install OS X Mavericks). When I select the boot disk, I'm taken to my Macintosh HD desktop anyway. It behaves as if I never selected the boot disk.
This happens with two different Mavericks install drives, one of which has been confirmed to work on other machines.
Partition map scheme for both drives is GUID and formatted to Mac OS Extended (Journaled).
Update: This is a Macmini2,1