I switched from an Ubuntu to the CentOS yesterday. But it seems I have messed my installation somehow, since OS will only start correctly if the USB with the installation is plugged in.Long story:   I've downloaded a live DVD CentOS 6 from the public repository, and using it with unetbotin created USB installation. That USB was used to install CentOS.During the installation, (only 1 ) hard disk was mounted under /sdb and USB installation was mounted under /sda. I think I selected to boot from the /sdb.Output of sudo fdisk -lDisk /dev/sdb: 1000.2 GB, 1000204886016 bytes249 heads, 62 sectors/track, 126540 cylindersUnits = cylinders of 15438 * 512 = 7904256 bytesSector size (logical/physical): 512 bytes / 512 bytesI/O size (minimum/optimal): 512 bytes / 512 bytesDisk identifier: 0x671e6467   Device Boot      Start         End      Blocks   Id  System/dev/sdb1               1       26533   204800000    7  HPFS/NTFS/dev/sdb2           26533       28557    15624193    5  Extended/dev/sdb4   *       28569      126540   756238336   83  Linux/dev/sdb5           26533       28557    15624192   82  Linux swap / SolarisDisk /dev/sda: 8086 MB, 8086618112 bytes37 heads, 13 sectors/track, 32836 cylindersUnits = cylinders of 481 * 512 = 246272 bytesSector size (logical/physical): 512 bytes / 512 bytesI/O size (minimum/optimal): 512 bytes / 512 bytesDisk identifier: 0x90909090   Device Boot      Start         End      Blocks   Id  System/dev/sda1   *           5       32837     7896064    b  W95 FAT32Wierd thing:if the USB stick is not connected to the machine, the boot screen will show up ( with both new installation of the CentOS and old Windows installation). But after I select either of them, after few seconds the machine will reboot. If the USB is connected, the same or similar boot screen shows up I OS starts normally.I've tried to reinstall GRUB using grub-install using /dev/sdb and /dev/sda options, but I wasn't able to make progress.Also, I don't understand purpose and content of the device.map file:(hd0)   /dev/sda(hd1)   /dev/sdbI tried removing /dev/sda from the device.map, which also doesn't help.grub.conf from /boot/grub/# grub.conf generated by anaconda## Note that you do not have to rerun grub after making changes to this file# NOTICE:  You do not have a /boot partition.  This means that#          all kernel and initrd paths are relative to /, eg.#          root (hd1,3)#          kernel /boot/vmlinuz-version ro root=/dev/sdb4#          initrd /boot/initrd-[generic-]version.img#boot=/dev/sdadefault=0timeout=5splashimage=(hd1,3)/boot/grub/splash.xpm.gzhiddenmenutitle CentOS (2.6.32-220.2.1.el6.x86_64)        root (hd1,3)        kernel /boot/vmlinuz-2.6.32-220.2.1.el6.x86_64 ro root=UUID=6cbcc55f-3342-4e85-a4c2-8355e73962f2 rd_NO_LUKS LANG=en_US.UTF-8  KEYBOARDTYPE=pc KEYTABLE=croat rd_NO_MD quiet SYSFONT=latarcyrheb-sun16 rhgb crashkernel=128M rd_NO_LVM rd_NO_DM        initrd /boot/initramfs-2.6.32-220.2.1.el6.x86_64.imgtitle CentOS (2.6.32-220.el6.x86_64)        root (hd1,3)        kernel /boot/vmlinuz-2.6.32-220.el6.x86_64 ro root=UUID=6cbcc55f-3342-4e85-a4c2-8355e73962f2 rd_NO_LUKS LANG=en_US.UTF-8  KEYBOARDTYPE=pc KEYTABLE=croat rd_NO_MD quiet SYSFONT=latarcyrheb-sun16 rhgb crashkernel=128M rd_NO_LVM rd_NO_DM        initrd /boot/initramfs-2.6.32-220.el6.x86_64.imgtitle Other        rootnoverify (hd1,0)        chainloader +1EDIT: more informationby deleting quiet and rhgb from the grub.conf I was able to see the error message :  No devices found .
Trouble booting CentOS
centos;boot;troubleshooting
It seems the problem was incorrect mapping of the hard disk.If the USB was plugged in during boot, hard disk was recognized as hd1; but if there wasn't the USB, it was recognized as hd0.Fixed the problem by changing grub.conf to use hd0.