China DOS Union

Ten Strategies for Saving Hard Drives

To effectively preserve data in the hard drive, in addition to regular backup work, one should also learn how to rescue the hard drive when it malfunctions or extract useful data from bad sectors to minimize losses.

1. The system does not recognize the hard drive
The system cannot boot from the hard drive, and cannot enter drive C from drive A. The automatic detection function in CMOS cannot find the hard drive. This kind of failure mostly occurs on the connection cable or IDE port. The possibility of the hard drive itself being faulty is small. By reinserting the hard drive cable or replacing the IDE port and cable for replacement tests, the location of the failure will be quickly found. If the newly connected hard drive is not recognized, a common reason is the master-slave jumper on the hard drive. If two hard drive devices are connected on one IDE cable, the master-slave relationship should be distinguished.

2. Failures caused by CMOS
The correctness of the hard drive type in CMOS directly affects the normal use of the hard drive. Now the machine supports the "IDE Auto Detect" function, which can automatically detect the type of the hard drive. When the hard drive type is incorrect, sometimes the system cannot boot at all, and sometimes it can boot but there will be read-write errors. For example, if the hard drive type in CMOS is smaller than the actual hard drive capacity, the sectors behind the hard drive cannot be read and written. If it is in a multi-partition state, some partitions will be lost. Another important failure reason is that since current IDE supports logical parameter types, the hard drive can use "Normal, LBA, Large", etc. If data is installed in a general mode and then changed to other modes in CMOS, a hard drive read-write error failure will occur because its mapping relationship has changed and the original correct hard drive position cannot be read.

3. Boot failure caused by the master boot program
The master boot program is located in the master boot sector of the hard drive, mainly used to detect the correctness of the hard drive partition and determine the active partition, and is responsible for handing over the boot right to the DOS or other operating system of the active partition. If this program is damaged, it cannot boot from the hard drive, but after booting from a floppy disk or CD-ROM, the hard drive can be read and written. The method to repair this failure is relatively simple. Using FDISK of a high-version DOS is the most convenient. When running with the parameter /mbr, it will directly replace (rewrite) the master boot program of the hard drive. In fact, the master boot sector of the hard drive is established by this program, and FDISK.EXE contains the complete master boot program of the hard drive. Although the DOS version is continuously updated, the master boot program of the hard drive has not changed. From DOS 3.x to the DOS of Windows 95, as long as a DOS boot disk is found to boot the system and run this program, it can be repaired.

4. Boot failure caused by partition table error
Partition table error is a serious error of the hard drive. Different degrees of errors will cause different losses. If there is no active partition flag, the computer cannot boot. But after booting the system from a floppy disk or CD-ROM, the hard drive can be read and written, and the active partition can be reset through FDISK for repair.
If a certain partition type is incorrect, a certain partition will be lost. The fourth byte of the partition table is the partition type value. The normal bootable basic DOS partition greater than 32MB has a value of 06, and the extended DOS partition has a value of 05. Many people use this type value to realize the encryption technology of a single partition. Restoring the original correct type value can make the partition return to normal.
There are other data in the partition table for recording the start or end address of the partition. The damage of these data will cause confusion or loss of the partition. The available method is to write back the backed-up partition table data, or obtain the partition table data from other hard drives of the same type and with the same partition status.
The recovery tool can use tools such as NU, which is very convenient to operate. Of course, DEBUG can also be used for operation, but the operation is cumbersome and has certain risks.

5. Failure of partition valid flag error
There is also an important part in the master boot sector of the hard drive, that is, its last two bytes: "55aa", this byte is the valid flag of the sector. When booting from the hard drive, floppy disk or CD-ROM, these two bytes will be detected. If they exist, it is considered that there is a hard drive; otherwise, the hard drive will not be recognized. Here can be used for the encryption technology of the entire hard drive, and can be restored by the DEBUG method. In addition, when the DOS boot sector has no boot flag, the system boot will display: "Mmissing Operating System". The convenient method is to use the following general repair method of the DOS system.

6. Boot failure caused by the DOS boot system
The DOS boot system is mainly composed of the DOS boot sector and the DOS system file. The system file mainly includes IO.SYS, MSDOS.SYS, COMMAND.COM, where COMMAND.COM is the shell file of DOS, and can be replaced by other similar files, but in the default state, it is a necessary file for DOS boot. In the DOS system carried by Windows 95, MSDOS.SYS is a text file, which is a necessary file for booting Windows, but when only booting DOS, this file can be not used. When the DOS boot is wrong, after booting the system from a floppy disk or CD-ROM, use the SYS C: command to transfer the system, and the failure can be repaired, including the boot sector and the system file can be automatically repaired to the normal state.

7. Read-write failure caused by the FAT table
The FAT table records the storage address of the hard drive data. Each file has a group of FAT chains to specify its cluster address. The damage of the FAT table means the loss of the file content. Fortunately, the DOS system itself provides two FAT tables. If the current FAT table is damaged, the second one can be used for coverage repair. But because the length of the FAT table and the address of the second FAT table of different specifications of the disk are not fixed, so when repairing, the correct position must be correctly found. Some tools such as NU itself have such a repair function, and it is very convenient to use. DEBUG can also realize this operation, that is, use its m command to move the second FAT table to the first table. If the second FAT table is also damaged, the hard drive cannot be restored to the original state, but the file data is still stored in the data area of the hard drive, and can be repaired by the CHKDSK or SCANDISK command, and finally get the *.CHK file, which is the sector data of the lost FAT chain. If it is a text file, the complete or part of the file content can be extracted from it.

8. Boot failure caused by directory table damage
The directory table records the file name and other data of the hard drive. The most important item is the starting cluster number of the file. The directory table has no automatic backup function, so if the directory is damaged, a large number of files will be lost. A method to reduce the loss is also to use the repair method of the CHKDSK or SCANDISK program, search for the *.CHK file from the hard drive. Because when the directory table is damaged, only the first cluster number is lost, each *.CHK file is a complete file, and changing it to the original name can restore most files.

9. Data recovery when the partition is deleted by mistake
After deleting the hard drive partition with FDISK, the data in the hard drive seems to have completely disappeared. When entering the hard drive without formatting, it will display as an invalid drive. If you understand the working principle of FDISK, you will know that FDISK only rewrites the content in the master boot sector (0 surface 0 track 1 sector) of the hard drive, specifically, it deletes the partition table information of the hard drive, and the data of any partition in the hard drive has not changed. You can imitate the repair method of the partition table error mentioned above, that is, find a way to restore the partition table data to restore the original partition and data. If the partition has been formatted, after restoring the partition first, you can restore the partition data by the following method.

10. Data recovery when the hard drive is formatted by mistake
In the high-version state of DOS, the FORMAT formatting operation establishes the disk information for restoring formatting by default, in fact, it copies all the content of the DOS boot sector, FAT partition table and directory table to the last few sectors of the disk (because the latter sectors are rarely used), and the content in the data area has not changed. Thus, by running the UNFORMAT command, it can be restored. In addition, DOS also provides a MIROR command for recording the current disk information for use in formatting or deletion recovery, and this method is also more effective.

Some of these methods are a bit outdated . For example, if you mistakenly format the hard drive, it's almost impossible to recover using UNFORMAT now, but FinalData can handle it very conveniently.

It is okay in the pure DOS environment, and unformat can also be used to recover. Actually, whether it can be recovered depends on two things: one is whether the information of unformat is saved, and the other is whether data has been written into it again. If data has been written into it again, then it is estimated that it cannot be recovered, or at least cannot be completely recovered, and maybe individual files can be recovered with other software.
If you encounter a misformatted disk or a mistakenly deleted file or a misoperation on the MBR, FAT, or boot sector, remember that the first thing to do is not to write any more data into it. If you still want to retrieve what you lost due to the misoperation, be sure to remember this sentence.
I have used L&F, which is very easy to use. I like it. I have heard of finaldata, but I haven't used it. After all, the times when such software is needed are relatively few. If you had to use it every day, then - oh, what's the fun? What's there to play with such a unlucky machine?