Mechanical failure. Clicking, beeping, scraping or a drive that does not spin up usually means failed read/write heads or a seized motor. We open the drive in the clean room, replace the head stack with a matching donor part and image the data sector by sector on dedicated hardware. With platter damage and helium drives this becomes a multi-stage process.
Firmware and electronics. Many drives are mechanically sound but are not detected or report the wrong capacity. The cause is a fault in the service area, the translator or the PCB. On current drives the board and the drive are paired through adaptive data, so swapping the PCB alone does not help. We transfer the ROM and adaptives and repair the firmware.
External drives. Many USB drives have the USB connector soldered directly onto the board, often with hardware encryption. Recovery then needs the original electronics or a decryption procedure.