Water or Fire Damage
Water or fire damage sounds like the worst-case scenario in this catalog, and sometimes it is. More often, though, the platters or flash chips inside are better protected than the case around them suggests — and the instinct to dry the drive out and try it is usually what causes the real damage.
What's actually happening
Water and fire damage a drive in different ways. With water, the real threat is corrosion — anything but pure distilled water carries dissolved minerals that leave a conductive residue as it dries, bridging connections on the circuit board that should stay isolated. That process begins fairly quickly once a wet drive is powered on, or even just left to air-dry without control. The platters or flash chips themselves, sealed away from the electronics on many drives, can survive full submersion completely fine. With fire, direct flame reaching the storage media itself is usually unrecoverable, but far more common is smoke and heat damage to the case and circuit board while the sealed internal chamber of a hard drive stayed protected long enough underneath.
How recoverable this really is
For water damage, often good, provided the drive isn't powered on while wet and is instead kept wet, or professionally dried and cleaned, before any power is applied — the electronics can be repaired or bypassed while the platters remain unaffected. For fire damage, the honest answer depends heavily on how close the flame or extreme heat actually got to the media itself, which usually can't be judged without a proper inspection.
The general approach
For a wet drive, keeping it wet — sealed in a bag rather than left to air-dry — until it can be properly disassembled and cleaned in a controlled environment prevents the corrosion that a few hours of uncontrolled drying would cause. For fire or smoke damage, the priority is getting the drive to a lab with a cleanroom, since even opening the case at home introduces the exact soot and debris contamination a cleanroom exists to prevent.
Do not, under any circumstance, power on a drive that's been wet — not "just to check," not once it looks dry. And don't attempt to open, disassemble, or even wipe down a fire- or water-damaged drive yourself. Both mistakes cause far more permanent data loss in this scenario than the original water or fire exposure ever does.
Practical, step-by-step guides for specific recovery software will be added here as separate linked articles.
This page describes the general situation, not a guaranteed fix for your specific case. If you'd rather have someone experienced take a look before you try anything, see what a hands-on evaluation looks like.