Electronics board. This is where all the work is done for the positioning system (VCM), data reads and writes, Analogue to Digital Conversion (reads) and Digital to Analogue Conversion (writes), timing signals, voltage control, cache memory and a whole host of error checking and other feats are performed.
It is a minicomputer whose sole job is data input, output and control for the hard disk drive. Each manufacturer has their own microcode hard coded into the processor or stored in ROM and extra information stored in RAM
When a circuit board fails, then is a close to 100% chance of recovery. The symptoms are usually that the drive will show no signs of life whatsoever, no spinning, no noises. We are able to extract critical information from ROM or NV-RAM on the PCB and reprogram a new PCB with this information enabling us to fully recover the data. In some cases we are even able to rebuild unique ROM content manually and get a drive into a state where the data can be read.