I thought it may be worth while to give an overview on data on a hard disk and how it works so that people have an understanding in case they suddenly find themselves loosing data on a disk.
When a hard disk comes out of manaufacturing it will have what is known as a factory low level format on it which basically ensures that it is clean and working correctly. When it is installed into a computer a partition is created or in some cases more than one partition can be created on a single physical disk. The parition is then formated to the operating system that is to be installed on it this could be XP or Vista, Windows 98 etc at this point 2 tables are created called FAT tables (file allocation tables). When data is written to a hard disk a pointer entry is put into the FAT table which basically acts as a look up table for where on the hard disk the start point and end point for the data is. (a map to the datas locatetion on the disk surface, like logitude and latitude on a map).
When you delete data or data is accidently deleted all that gets removed is the entry point for the data and not the actual data itself. A standard undelete software package such as Nortons Disk Doctor (or Norton utilities as it may now be called) will easily scan the hard disk and rebuild these tables recovering data.
When you format a hard disk all that is formatted are these 2 tables, again there are software packages out there that will recover these tables and your data will be fully recovered. (there are many people currently being held at Her Majestys pleasure who will bare witness to this when they thought they were safe).
Where things get tricky is that if you have deleted data (or the entry in the tables) and new data is written to the hard disk and it overwrites what was allready on the disk surface most commercial packages will not recover from this. This is not the data entry in the tables but the actual data on the disk surface. If the new data is written to a different area then your original data is still intact and can be recovered. Problem is it is upto the computer to decided where it writes this new data and if it will over write what was there. So as soon as you know you maybe over writing something you have deleted stop the data write. Try a recovery program and you maybe lucky.
When a hard disk looses its partition again data is still on the hard disk and commercial programs can recover the partition, all they will do is re-construct the original partition based on the information on the disk. Again data will be recoverable
Where data is more or less unrecoverable is when a factory low level fomat is carried out, this will make data recovery very hard and expensive. data is written to the harddisk using a magnet (so dont put hard disks near items with a strong magnetic charge they will erase data) so even with 10 or 12 factory low level formats and data written on at each stage previous data is still possible to be recovered by very specialist companys who recover the magnetic signature that will allow data to become visable.
So data can be recovered nearly all the way down the line, so dont panic if it happens to you. At the end of the day it comes down to how much you are willing to pay to get data recovered. Some packages are free as shareware and some more advanced ones you have to pay for.
If a hard disk dies (they all will eventually) remember your data is written to the surface of the disk and not to the electronics on a hard disk, swapping over the electronics from another working hard disk that is the same will allow in most cases data to be accessed again. The only way to prevent data from being recovered on a hard disk is to smash it with a hammer.
I hope this helps to explain to people how to look at going about data recovery.
I will do another one on hints and tips to data storage, protection and recovery