How Donor Drives Are Matched for Head Swaps
A head swap requires transplanting the entire Head Stack Assembly (HSA) from a compatible donor drive into the patient drive (the drive with failed heads). The donor heads must be mechanically compatible with the patient's platters and electronically compatible with the patient's firmware and preamp circuitry. Matching by model number alone is not sufficient. Hard drive manufacturers produce the same model number across multiple hardware revisions, with different head components, preamp chips, and firmware variations within a single model line.
Firmware Revision Matching
The drive's firmware revision is the primary matching criterion. The firmware controls how the drive communicates with the heads: signal timing, read channel calibration, servo decoding parameters, and write current profiles. Heads from a donor with a different firmware revision may not work because the patient drive's firmware expects specific electrical characteristics from the heads that the donor heads do not provide.
Firmware revisions are printed on the drive label. For Seagate drives, this is a four-character code (e.g., CC26, SDM1, 0001). For Western Digital, the firmware revision is part of the extended model number. The firmware revision indicates the generation of controller code and, by extension, the generation of head technology the firmware is calibrated for.
Within a firmware revision, there can be sub-revisions that affect compatibility. Two drives with firmware "CC26" manufactured six months apart may have different micro-code patches. In most cases, same firmware revision is sufficient. In some Seagate Rosewood drives, even drives with the same firmware revision but from different manufacturing sites (identifiable by the DCM code) have different head compatibility.
Head Map and Head Count
A drive's head map specifies which heads are installed and active. A two-platter drive can have 2, 3, or 4 heads depending on the capacity variant. A Seagate Rosewood ST2000LM007 (2 TB) uses 4 heads across 2 platters. The ST500LM030 (500 GB) uses 2 heads on 1 platter. Both are "Rosewood" drives, but their head assemblies are physically different.
The head map is stored in the drive's firmware and defines which physical head positions are active and how the firmware addresses them. The donor must have the same head map as the patient: same number of heads in the same physical positions. A 3-head donor HSA cannot be used in a 4-head patient drive because the firmware expects to address a head that does not exist, and the physical mounting may differ.
| Matching Criterion | Where to Find It | Why It Matters |
|---|---|---|
| Firmware revision | Drive label, IDENTIFY DEVICE response | Firmware calibrates read channel and write current for specific head characteristics |
| Head count / head map | Firmware SA modules, capacity variant | HSA must have same number and position of active heads |
| Preamp chip model | Visible on HSA flex cable PCB | Signal path between heads and controller must be electrically compatible |
| Manufacturing date / site | Drive label (date code, DCM, country) | Drives from the same batch use the same head components and calibration |
| Platter count | Drive label (capacity + model), physical inspection | HSA arm count must match platter count |
Preamp Chip Compatibility
The preamp is a small IC mounted on the HSA flex cable, inside the sealed drive cavity. It amplifies the microvolt signals from the read heads and drives the write current to the write heads. The preamp model must match between donor and patient because the controller's read channel is calibrated for the specific signal characteristics of that preamp.
Preamp models can be identified by the marking on the chip (visible when the drive is opened) or inferred from the firmware revision and drive family. Common preamp manufacturers include Texas Instruments, Broadcom (Avago), and Renesas. Within the same drive model line, a preamp change usually coincides with a firmware revision change.
Adaptive Parameters and Post-Swap Calibration
Every hard drive generates a unique set of adaptive parameters during factory self-scan. These parameters record the specific calibration data for the drive's individual heads: optimal read channel settings, write current values, fly height compensation, and servo tracking offsets. Adaptive parameters are stored in the drive's System Area on the platters and backed up in the ROM chip on the PCB.
After a head swap, the patient drive's adaptive parameters no longer match the installed heads. The parameters were calibrated for the original heads, not the donor heads. In some cases, the drive can still read with minor degradation. In other cases, the mismatch prevents the drive from reading its own System Area, causing it to fail initialization.
PC-3000 can modify adaptive parameters after a head swap. The technician can load the donor drive's adaptive parameters into the patient drive's firmware, aligning the calibration with the installed heads. This is not always necessary (some drive families tolerate mismatched adaptives well enough for imaging), but it improves read stability and reduces errors on drives where the mismatch causes issues.
Manufacturing Date and Factory Site
The closer the donor is to the patient in manufacturing date and production site, the higher the compatibility probability. Drives manufactured in the same batch at the same factory use the same component lots: same head wafer, same preamp batch, same platter lot. Component-level consistency within a production batch is high.
Seagate encodes manufacturing information in the DCM (Drive Configuration Matrix) printed on the label. The DCM indicates the head type, preamp, motor, and other configuration details. Two drives with the same model number and firmware revision but different DCMs may have incompatible heads.
Western Digital uses a board part number and production date code on the label. The board part number indicates the PCB revision and, by extension, the expected head/preamp configuration. Production dates within a few months of each other are preferred.
Same model number does not mean compatible donor.
A Seagate ST2000LM007 with firmware CC26 from January 2018 and an ST2000LM007 with firmware SDM1 from 2020 are different hardware generations despite sharing a model number. The heads, preamp, and firmware are different. Using the SDM1 drive as a donor for the CC26 patient will not work. Matching requires firmware revision, head map, and manufacturing proximity.
How Labs Maintain Donor Inventory
Professional recovery labs maintain an inventory of donor drives organized by manufacturer, model family, firmware revision, head map, and manufacturing date range. Common drive families that fail frequently (Seagate Rosewood, WD Blue/Green 2.5", Samsung Spinpoint M8) are stocked in higher quantities.
When a patient drive arrives, the technician identifies the required donor specifications from the drive label and firmware. If the lab has a matching donor in stock, the head swap can proceed immediately. If not, the lab sources one from supplier networks, which may take 1-5 days depending on the drive's rarity.
Donor drives are purchased specifically as parts inventory. They are functional drives that have been verified to read and write normally. Using a donor from a failed drive (e.g., a drive that had bad sectors but working heads) is possible but risky: the heads may be degraded even if they currently function.
Frequently Asked Questions
Why can't I just buy the same model hard drive for parts?
Matching by model number alone is insufficient. Manufacturers produce the same model across multiple hardware revisions with different head components, preamp chips, and firmware. The firmware revision, head map, preamp compatibility, and manufacturing batch all affect whether donor heads work with the patient drive's platters and firmware.
How many donor drives does a lab typically try?
Most successful swaps use the first well-matched donor. When matching criteria are tight (rare head configuration, specific firmware sub-revision), a lab may test two or three donors. Each attempt involves installing donor heads, powering on under PC-3000 control, and checking read stability. Labs maintain inventories sorted by firmware revision, head map, and manufacturing date to minimize trial-and-error.
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