I've searched on the web about this problem and it's usually solved by installing the appropriate drivers. Problem is I'm on a Mac.
When I plug in my device, it can be discovered when I run
adb devices. I run
adb reload bootloader and it goes into download mode (the mode you get when you hold Volume Down + Home button when powering up). Is this correct? I assume it is.
Anyway, when it is in this stage,
adb devices or
fastboot devices yields nothing.
I have tested this on i9100 and i9505.