Whether you have come across the case that you can not start the computer from the floppy or hard disk? When computer’s hard disk partition table infected by virus.usually the computer can start from a floppy disk. However, in severe cases, not only can not boot machine from the hard disk ,but also from a floppy disk. Some viruses can deadlock the hard disk,in this case the only way to solve the problem is unlock the hard disk logic deadlock.Â
Hard disk lock usually is a hard disk partition table tricks, so first of all we should be aware of the hard disk partition table. The hard disk partition table is located 0 cylinder 0 head 1 sector, more than 200 bytes in front of the sector is the main boot process, followed  64 byte started from the 01BEH is the partition table. A total of 64 bytes of the partition table is divided into four columns, each column 16 bytes, is used to describe a partition. If use DOS’s FDISK program partition, a maximum of only two columns, the first column describes the basic DOS partition,the third columns describe the extended DOS partition. The meaning of partition table 1 column structure and each byte is as follows:
00H-byte flag events, activities DOS partition for the 80H, others are 00H.
01H- magnetic head number of the head logical 0 sector.
02H- secter number of the Logic 0 sector in the cylinder.
03H- cylinder number of Logic 0 sector.
04H-partition type flag.
05H-1 the last head magnetic head number of the sector in this partition.
06H-the last sector number of sector.
07H-the end cylinder number of the cylinder.
08H-hard disk sectors in the district prior to the total number of double-word, said.
0CH-the total number of sectors in this area, count from the logical  0 sector, does not contain hidden sector, with pairs of words indicated.
Sector number indicated with the No. 6 ,the cylinder with the No. 10 . the maximum bytes of the sector is actually the two highest 2 of the cylinder number.
The last two bytes of the partition table partition table is a valid symbol, if changed will not boot from the hard disk, which is a simple way to lock the hard disk. The solution is to boot from a floppy disk, after started the hard disk is still available. Using Debug or Noratn in Diskedit software will drive the partition table,recovery a sign of the hard disk, there is no problem to boot from the hard disk.
Another way to lock the hard drive is do something to partition parameter, and if all partition parameter becomes 0, then the boot partition is unable to find argument,there is no way to start from the hard disk, just do not recognize the hard disk boot from a floppy disk , if you type the drive letter C and press ENTER, a prompt appears Invalid driver specification. However,the DOS-Debug can still read the hard disk 0 cylinder 0 head 1 sector of the content with A disk, modify and then write 0 cylinder 0 head 1 sector, restart the machine there will be no problem.
If random change the partition table parameters to other parameters, it may be cann’t started by DOS system disk, press F3 to withdraw there will be prompt a memory allocation error and can not load DOS command, the system crashes , but using a formatted floppy disk into the system can be successfully launched, as long as Debug, you can still modify the parameters of the partition table to original.
Terrible thing is that if you are unfortunate enough to partition the table into a circular chain of argument, that C drive partition to the next point to D drive, D drive partition then the next one to point C, so that cycle continues,whether you use a floppy disk or a hard drive will no capability to start the machine because of the endless  logical drive reading,not start is caused by hard disk, even if you carry it to the other computer, also can not use, so the hard disk is locked completely.
A complete hard drive lock program is to rewrite 0 cylinder 0 head 1 sector of the boot process, and destroy the partition table, or deliberate creat  a loop partition table,but put the real parameters of the hard disk partition table and bootloader on the other hidden sector and protect them, if you start it with the wrong password, you can not start the machine.
