Consider the situations which cause a computer to hang up while booting or hang up due to data loss.
(this could happen if, for example, FDISK is used and an improper partition is set to active).
The loader will try to boot from there, fail, and try to boot again from other devices like a floppy. If fails to boot again, an error message like "Non-System Disk or Disk Error" will appear.
It contains all the necessary information about the partition so that if recovery software finds it, it can reconstruct the partition entry in the Partition Table. (see Partition Boot Sector topic for details).
In this case, a new partition would exist instead of the original one. Everything would work properly with the exception that you could not go back to the original partition if important data is still there. If you've created a MBR, a Partition Table, or a Volume Sectors backup beforehand (as can be done with Active@ Partition Recovery and Active@ UNERASER for example), you can virtually restore it and look for your data (if it has not yet been overwritten by new data).
Some advanced recovery tools also have an ability to scan disk surfaces and try to reconstruct previously deleted partition information from the pieces of left over information (i.e. perform virtual partition recovery). However it is not guaranteed that you can recover something.
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