Initial Bootloader > Flash Drive. Warning. If not used carefully this process can be dangerous

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Winfred Garrett
6 years ago
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1 Initial Bootloader > Flash Drive Warning If not used carefully this process can be dangerous Running the script used in this article with an incorrect argument can cause loss of data and potentially damage the computer. The script with this article makes sector-to-sector copies to the target media which cannot be undone. To ensure that the computer is not compromised, follow the instructions in this article with care and ensure that the argument passed to the script is not in doubt. It is recommended to make backups of valuable data on the computer before performing operations that cannot be undone. Introduction This article explains how to populate a flash drive (the target media) with a bootloader. The bootloader used in this article is GRUB (GRand Unified Bootloader). GRUB is self-propagating and once it exists on media in one device it can be used to install itself onto another target. However, it is not possible to install GRUB using a Windows executable, so the script in this article populates the target with an initial image using dd to perform a sector-tosector copy of a GRUB image.

2 Prerequisites The following hardware is required: one flash drive The following Windows programs must be installed: Tar gzip dd It is also essential that the computer can boot from a flash drive. After the computer completes its power on self test, it searches for bootable storage devices and boots from the first one it finds. The order in which the computer searches for devices is controlled in the ROM BIOS and the BIOS settings need to be configured to instruct the computer to search for bootable flash drives before searching for bootable hard disks. Computers vary widely in how this is done. Many display a message shortly after the computer is turned on indicating which key to press to enter BIOS configuration (typically the esc or F10 key). Others require a configuration floppy disk or CD to configure the BIOS.

3 Download Download initial_flash from helpcentreonline.com. This will save a file named initial_flash.tar.gz. Note the directory initial_flash.tar.gz is downloaded to: this will be referred to as the download directory ([download]). Installation 1. Collect information. 1. Note the target device mount identifier. In Windows, flash drives are represented by drive letters. The drive letter can be found by starting Windows Explorer and then plugging the flash drive into a USB socket. Note the drive letter used to represent the flash drive. The drive letter is referred to later in this article as the target identifier. Leave the flash drive in the USB socket. If devices are connected to or disconnected from the computer, the target identifier may change. For this reason it is important to note the target identifier each time the process in this article is followed and complete the process before connecting or disconnecting any devices. 2. Note the target device ([target device]). Start a command prompt and type the following command (do not type the > character - it is the prompt): > dd --list A list of devices is returned. Look through the list and find the reference to the target identifier. An example of a reference in the output is as follows: \\.\Volume{1def118c-6a20-11dd-b d }\ link to \\?\Device\Harddisk1\DP(1)0-0+b removeable media Mounted on \\.\f: In this example, the target identifier is f and the target device is Harddisk1.

4 2. Backup target. 1. Operations performed in this article overwrite the master boot record and destroy all existing partitions on the target flash drive. One small partition is created on the drive and the rest of the device is marked as unused. This means that much of the flash drive is unusable without rebuilding the partitions, and even if partitions are rebuilt, the master boot record's code area is overwritten with GRUB's initial bootstrap program, so the flash drive may still not be bootable if GRUB is removed. To preserve any existing data on the drive and make it fully usable again without altering its partitions, it is possible to backup the entire drive using the following command at the command prompt: > dd if=\\?\device\[target device]\partition0 of=temp.img bs=1m When typing the command replace [target device] with the target device value noted earlier. For example, if the target device is Harddisk1, type the following: > dd if=\\?\device\harddisk1\partition0 of=temp.img bs=1m Before typing the above command it will be necessary to navigate to the directory the backup image is to be stored in. Do not type the > character - it is the command prompt. The data can later be restored to the flash drive using the following command: > dd if=temp.img of=\\?\device\[target device]\partition0 bs=1m 3. Create sources directory ([sources]). 1. Create a directory from which the flash drive installation will be performed. For example, a suitable directory could be c:\unix\source. This directory will be referred to as the sources directory.

5 4. Unpack sources. Start a command prompt and type the following commands. (Do not type the > character - it is the command prompt.) 1. > copy [download]\initial_flash.tar.gz [sources] Copy the sources from the download directory to the sources directory. When typing the command replace [download] with the path name of the download directory including the drive letter and replace [sources] with the path name of the sources directory including the drive letter. For example, if initial_flash.tar.gz was downloaded to d:\download and the sources directory is c:\unix\source, type: > copy d:\download\initial_flash.tar.gz c:\unix\source 2. > [sources drive]: Change to the drive containing the sources directory. When typing the command replace [sources drive] with the drive letter of the drive containing the sources directory. For example if the sources directory is c:\unix\source, type: > c: 3. > cd [sources] Change to the sources directory. When typing the command replace [sources] with the path name of the sources directory excluding the drive letter. For example if the sources directory is c:\unix\source, type: > cd \Unix\source 4. > gzip -d initial_flash.tar.gz Decompress the compressed file. 5. > tar -xf initial_flash.tar Extract the archive.

6 5. Install. 6. > cd initial_flash 1. > install [target device] 6. Clean sources directory. Install the GRUB image. When typing the command replace [target device] with the target device. For example, if the flash drive is represented by Harddisk1, type the following: 1. > cd.. > install Harddisk1 2. > del initial_flash.tar 3. > rd initial_flash /S

7 7. Test installation. 1. Reboot the computer with the flash drive still connected. If the installation has been successful the computer reboots with the following screen: GRUB version 0.93 (640K lower / 3072K upper memory) [ Minimal BASH-like line editing is supported. For the first word, TAB lists possible command completions. Anywhere else TAB lists the possible completions of a device/filename. ] grub> As indicated in the prerequisites section, the BIOS must be configured so that flash drives have priority over hard disks in the search order. Script 2. Remove the flash drive and reboot the computer. The following script is used: Unplug the flash drive and then at the grub> prompt type: grub> reboot install.bat is run from the command prompt to perform the installation. install.bat The install.bat script is run from the command prompt to perform the installation. It accepts the target device as an argument and copies the GRUB image to the flash drive.

8 Further Comments Script To understand the process followed in this article, it is worth examining the script. The installation is performed by the following command in the install.bat script: dd if=grub.img of=\\?\device\%1\partition0 bs=1m This command uses dd to copy the contents of file grub.img to the target flash drive in blocks of 1 megabyte. dd is not the same as the DOS copy command which copies one file to another. dd streams a specified quantity of data from a defined point in a source to a defined point in a target without regard to any content that may already be in the target. The source and target may be files or devices. In the above example all data is streamed from the beginning of file grub.img to the beginning of a flash drive. The first sector of a flash drive contains the master boot record, which is overwritten, and a small partition is copied. The dd command enables an entire image to be created on the target, which may contain data in a variety of forms, including: a single file the entire contents of a device an operating system a copy of a master boot record. Flash Drive Contents The image copied to the flash drive is in FAT32 format, so it can be viewed from Windows Explorer. The drive contains the following files in directory \boot\grub: stage1 stage2 fat_stage1_5 In addition, stage1 is also embedded in the drive's master boot record as the initial bootstrap program. When the computer starts, stage1 is loaded from the master boot record and it loads fat_stage1_5, which understands the FAT32 file system. fat_stage1_5 then loads stage2, which contains the user interface and functionality in GRUB.

Initial Bootloader. On power-up, when a computer is turned on, the following operations are performed:

Initial Bootloader. On power-up, when a computer is turned on, the following operations are performed: Initial Bootloader Introduction On power-up, when a computer is turned on, the following operations are performed: 1. The computer performs a power on self test (POST) to ensure that it meets the necessary