Electronics: this really is a repairWhere a drive shows absolutely no sign of life, the most likely cause is that the circuit board has failed: a shorted TVS diode, electronics that have given out, or, on a USB drive, a bridge chip that has stopped responding. Each of those is repair work in the plain sense, and typically the cheapest news anyone here gets to give.
Firmware: mendable with the right kitIf one module in the service area is damaged, or the translator is corrupted, a mechanically perfect disk will identify itself as 0GB, or disappear from the bus completely. Reaching that level takes purpose-built hardware. No downloadable tool can get close.
Mechanics: a single sweep and no moreA seized spindle freed, a donor head stack fitted, heads lifted off a surface they had settled onto: in every case, what that buys is exactly the working time needed for one complete copy, and not a moment longer. Outliving the job is not what the mechanism is for.
Swapping the board stopped working long agoMeasurement only happens once, during manufacture, and the resulting values are stored in a ROM soldered onto the circuit board that shipped with that mechanism. Fit a board from a different drive and the heads give you noise instead of data. Your own ROM has to move across to the new board, and that needs a soldering iron and a programmer.