AHCI specs say about the device detection field:
Device Detection (DET): Indicates the interface device detection and Phy
state.
0h - No device detected and Phy communication not established
1h - Device presence detected but Phy communication not established
3h - Device presence detected and Phy communication established
4h - Phy in offline mode as a result of the interface being disabled or
running in a BIST loopback mode
The "Software Initilaization" section also mentions "If PxSSTS.DET
returns a value of 1h or 3h when read, then system software shall
continue to the next step, ..."
This makes me think that 1h means "tried to detect device but didn't
found one" and 0h means "device detection not finished yet".