It is not a good idea to immerse a hard disk in water. The hard drive casing is not impermeable and its filtration capabilities stretch to the removal of contaminants in the air but will do nothing to prevent the ingress of any but the most minor attempts by water to enter the system.
Sadly, we do not always get the choice. When the heavens open, or a hurricane brings the sea on to the land, the immediate priority of “getting the **** out of here” tends to leave the data storage devices at the mercy of the elements and it is only once the aquatic problems subside that the consequences are truly revealed.
Initially water will affect the electronics, so starting up the drive will result in electricity going to places where you don’t really want it to and maybe even a puff of smoke or two, if the device has been submerged then the problems will stretch further.
Drying the disk out might not be enough. If water has made it into the HDA then the subsequent problems are serious. Usually the water associated with this type of problem is from floods, and so contains a whole raft of impurities. (note some of these impurities are also hazardous to human health as well as drive operation). https://precisionfinaldrives.com/
These impurities will coat the surface of the disk platter within the hard drive unit, and also render the read/write heads unusable. Attempts to run the drive once the moisture has finally gone might result briefly in success (though rarely), but inside the device damage will be taking place that will shortly result in the destruction of data.
The drive heads rely on air flow over a smooth shiny surface, and a rough dirt covered one will eventually result in contact between the heads and a platter that could be rotating at 10,000 rpm.
We have been sent disks where the damage does not seem server at first but on close inspection of the data surfaces of the platter there are scratches and rings where the heads have marked the surface as it passes beneath, often there are grooves in the surface caused by a sever contact.
What not to do?
Don’t take chances with your life or health.
Attempting to get equipment from, for example, a flooded basement is a very dangerous act. Flood water is not usually clean, so can carry disease and can also hide obstacles that could leave an untrained person becoming trapped or even drowning.
Leave the heroics to the professional, when they have done their work then you can do yours.
What Can Be Done
To deal with a water damaged hard drive great care must be taken. Check first that the data from the disk is needed, if you can find it elsewhere then a whole load of pain can be avoided.
To begin with the whole assembly be thoroughly cleansed, there is not room for taking a chance as any remaining residue could cause a serious problem.
Usually the heads must be replaced. Attempting to clean heads is a last resort option only to be attempted where the hard disk is long-term obsolete and no replacements can be obtained.
In some cases the effect of water and debris will result in problems with the spindle bearings and the disk drive platter will no longer rotate. This is probably the most severe problem as then the platter(s) have to be transferred to a new working housing. With single platter drives this is not too severe, but with multi-platter disks the chances of losing the precision alignment between the platters is great and with it any chance of recovering the data.
The prognosis is usually good if the recovery attempt is made soon after the water damage and does not involve just plugging in the disk to see what happens. We have seen many disks where the opportunity to recover the data has been lost by the disk being operated without first going through a cleansing process, the result being that the heads and platter become damaged, often terminally.