Install Ubuntu 15.04 on VirtualBox VM

virtual hard disk

When installing Ubuntu into a virtual machine (such as Virtual Box), you create a virtual hard drive to hold the virtualized OS and its data. To the host operating system, this virtual hard drive is just a file on a real hard drive, but the guest operating system can use it as if it were a real drive to partition it and format it according to the needs of the guest. This will keep the partitions and filesystems on the real drive intact.

Windows devices

In Windows , hard drives or their partitions are assigned letters A: B: C:, ...regardless of where the partitions are located . This is different in Unix and Ubuntu.

Device files for Unix and Ubuntu

For each attached block device (and each virtual hard drive in the VM), we will create a device file . Modern SCSI or S ATA d for incentives will be named /dev/sdfollowed by the serial number depending on which they are registered. So the device file for the first registered drive will be named /dev/sda, the second /dev/sdb, the third /dev/sdcand so on.

After partitions are added to the drive, they will be numbered consecutively /sda1, /sda2, /sda3, .... Therefore, we can easily see which hard drive a given partition is located on.

Grub and boot devices

Now to install the main boot loader (Grub) required to boot Ubuntu, we can further specify on which drive it should be installed. This only matters if you have multiple drives attached to your computer (or virtual machine).

In your case, you probably just created a virtual hard drive (though there could be more) and then named it /dev/sda. This drive is where you want to install Grub and OS.

Note that in a virtual machine, this is still a virtual hard drive, not the bare metal drive where the host OS resides.

In a word

On a real computer where we may have to choose from several operating systems, we may have to note that the bootloader is indeed placed on the first bootable drive. It replaces any other (eg Windows) Master Boot Record that existed before the start. Only then can a dual-boot system of the operating system be selected at boot time. If for any reason we don't want this, we can tell Grub to install it on another drive (or partition) if available.