Drives with corrupted firmware (yes, firmware is stored on the disc, the controller board only has a bootloader in it's ROM) broken heads, mis-aligned heads, bad hydro bearing on the spindle motor, scratches on the surface of the negative LBA area (firmware/calibration data/absolute bad sector maps) will be not detectable so a computer will be unable to access anything on the drive besides the controller board (which in that state will only reply to vendor specific commands).
These vendor specific commands could be used to repair a corrupted firmware but then all the other defects I mentioned are of mechanical nature.
SCPH-10000_GH-001 SCPH-15000_GH-003 SCPH-18000_GH-008 SCPH-30001_GH-005 SCPH-30000_GH-016(V4) SCPH-30001_GH-010(V4)
2xSCPH-10190, 2xSCPH-10350, 2xSCPH-10280
"**** j0 hackers!"
-Sjeep (As seen on TOXIC OS ELF...)