For CentOS installation, do not set the hard disk name as grub. The system is installed in the wrong place.

First, read the following concepts: the IDE hard drive starts with hd (now basically disappears, except for some older machines), and the SCSI hard drive starts with sd (now popular sata ). Floppy disk starts with fd (can be said to disappear directly ). The name is not the same as that of linux. Is counted from 0. (Sd0, 0) (why does it start from 0, because many foreign things start from 0 ). Disk C. (Hd0, 4 ). Disk D. Of course, this refers to (the first logical partition, if the D disk is also the primary partition, it should be written as sd0, 1) the first hard drive of the system should first read the following concepts:
IDE hard drive starts with hd (now basically disappears, except for some older machines), and SCSI hard drive starts with sd (now popular sata ). Floppy disk starts with fd (can be said to disappear directly ). The name is not the same as that of linux. Is counted from 0. (Sd0, 0) (why does it start from 0, because many foreign things start from 0 ). Disk C. (Hd0, 4 ). Disk D. Of course, this refers to (the first logical partition. if the D disk is also the primary partition, it should be written as sd0, 1). The first hard drive of the system is represented as (sd0 ), the first partition on the disk is represented as (sd0, 0). That is to say, the hard disk is represented in the form of (hdx, y). x and y are counted from 0, x indicates the hard disk number, and y indicates the partition number. Since there can only be four primary partitions, the four primary partitions on the first hard disk are used (sd0, 0 )~ (Sd0, 3). logical partitions are counted from (sd0, 4), that is, the first logical partition is used (sd0, 4), and the second logical partition is used (sd0, 5) and so on. Generally, the hard drive of the machine is a primary partition, while the rest are logical partitions. Therefore, disk C is represented by (sd0, 0), and disk D is represented by (sd0, 4. The disc is represented by (cd), and the first soft drive is represented by (fd0. File representation: (,)/path/to/file in Linux, for example, y in ubuntu, (sdx, y) starts from 1. The first logical partition is represented by (sd0, 5), and the second logical partition is represented by (sd0, 6 ).
Device Lilo Grub
Sata master sda, sda1, sda2 (sd0), (sd0, 0), (sd0, 1)
Sata slave sdb, sdb1, sdb2 (sd1), (hd1, 0), (sd1, 1)
Sata master sdc, sdc1, sdc2 (sd2), (sd2, 0), (sd2, 1)
Sata slave sdd, sdd1, sdd2 (sd3), (sd3, 0), (sd3, 1)
Where to install the image
1: Local cd/dvd
2: Hard disk device
3: nfs shared directory (similar to sharing with Windows)
3: url (that is, install from the network)

Here it means:
Install centos from the image in the partition and directory. if you do not see your device below, you can configure it by F2. here/dev/sda4/indicates that it is an extended partition of the USB flash drive, you can write the code sd () to indicate that you may doubt why the USB flash disk is the first hard disk. it is very simple that the system starts from the u, and the first one is to recognize the USB flash disk, so what is the following:
Sdb1 sd () is the primary partition of drive C.

Sdb2 sd () is the primary partition of the D disk.
Sdb5 sd () is also why the logical partition of the drive ranges from 1 to 4: The four primary partitions of the hard drive are used (sd0, 0 )~ (Sd0, 3). logical partitions are counted from (sd0, 4 ).
Presumably, everyone in sdb6 knows what it means.

This is another step. I believe that new users do not know how to choose from it. if it is a USB flash disk or the default one, it may lead you to unlimited installation, or prompt grub>, this step is to install grub. during grub installation, it is divided into two parts: one is written to mbr, and the other is written to boot partition/boot. If no/boot is installed, it is written to/partition, if there is no choice, write it to the USB flash drive, that is, sda.