ATA registers: what they mean

Link Register

It's only enabled when port powered on, device presence detected and PHY communication established.

Status Register

This register contains hard drive status information. It is updated after every single command sent to the drive.

ERR: means last command failed to execute. In this case the Error register contains more details on the specific error.
INDX: obsolete, used to trigger after each spindle revolution
CORR: obsolete, used to trigger after a bad sector was automatically corrected by ECC
DREQ (Data Request): is asserted when hard drive wants to exchange data with the host controller (in either direction)
DRSC (Device Seek Complete): is obsolete; always asserted on modern hard drives
FAULT (Write Fault): is obsolete
DRDY (Device Ready): is obsolete; always asserted on modern hard drives
BUSY: indicates that the hard drive is busy executing a command OR initializing (after power on or reset)

Error Register

Error register provides more details if the last command failed. This register is only valid when ERR bit of the Status Register is asserted.

AMNF: means Address Mark Not Found (usually occurs on failed read attempt)
T0NF (Track 0 Not Found): obsolete
ABRT: command aborted (unsupported command or other failure)
IDNF: sector ID not found (usually occurs on failed read attempt)
UNC: uncorrectable read error; the hard drive was unable to read data even after applying ECC recovery algorithms
ICRC (Interface CRC error): there was CRC error while transferring data between host and the hard drive (usually indicates bad interface cable)