Choosing between /dev/sda and /dev/sdb for boot loader installation

I am a Windows 10 user and I would like to try Ubuntu 18.04.3 LTS. I use Windows 10 to my 595 GB disk 0 (C:) and I wish to install Ubuntu on my empty 595 GB disk 1 (D:). My aim is to select which operating system to use on every startup. Therefore, I shrunk disk disk 1 (D:) and created an unallocated 200.20 GB partition in order to install there Ubuntu. I booted into Ubuntu using a USB Ubuntu bootable drive. In the partitions table, I clicked the 214 gb free space, and chose the following partitions for installation (I haven’t made any change yet or proceed to the installation).

/dev/sda   TYPE        SIZE    USED    SYSTEM /dev/sda1  ntfs       209 MB   42 MB  Windows 10 /dev/sda2  ntfs       424 GB  108 MB /dev/sda5  swap         9 GB /dev/sda6  ext4/       20 GB /dev/sda7  ext4/home  185 GB  /dev/sdb /dev/sdb1  ntfs       639 GB  581 GB  /dev/sdb2  ntfs       300 GB  448 MB 

You can see that I partitioned the free space to /dev/sda5, /dev/sda6 and /dev/sda7.


Questions

  1. How can I know which drives are /dev/sda and /deb/sdb? Since Windows 7 is installed on disk 0 (C:) and having used already 543 GB there, while disk 1 (D:) is empty, I suppose that /dev/sdb is disk 0 (C:) and that /dev/sda is disk disk 1 (D:). Is that correct? Furthermore, as the free space which was in disk 1 (D:) became /dev/sda5, /dev/sda6 and /dev/sda7, I assume again that disk 1 (D:) is /dev/sda. Are my thoughts correct?
  2. a) May someone explain to me what is the device for boot loader installation? Do I already have such a device? If so, how can I find which it is?

    b) I must choose to select either /dev/sda or /dev/sdb. The installer defaulted to dev/sda. Does it matter which I choose? Will a wrong choice affect my windows installation or files on disk 0 (C:)? Is it possible that I won’t be able to boot on Windows 10 if I choose wrong?

  3. In the worst case that something goes wrong with the boot loader installation, can I fix that problem booting from my USB bootable drive or I will have to reinstall Windows 10?

Note: There are similar questions here and here. The first link troubles me, the second link doesn’t give details for the boot loader installation, and both links don’t answer all my questions.

Thank you in advance for any help!

boot-repair fails with ‘No known boot loader is installed in the MBR of /dev/sda.’

I’m running a Windows 10/ubuntu dual boot system and have successfully used grub as boot manager until today. However, today the system unexpectedly failed to display grub at startup and instead booted to windows right away. I verified in UEFI that linux has higher boot priority, yet the problem persists. I also created a ubuntu live boot medium and ran bootrepair withit, albeit without success. The pastebin can be found here:

http://paste.ubuntu.com/p/t4SxpDWPGp/

I admit not being able to interpret this log in full. Any pointers as to where the underlying problem may lie are appreciated.

Additional info: I tried both with activated and deactivated secureboot, without any effect.

Can’t mount back OS disk partition (no /dev/sda)

I was trying to expand my Azure VM’s OS disk and I’ve unmounted /dev/sda1 which is OS partition in order to do the steps mentioned here: https://docs.microsoft.com/en-us/azure/virtual-machines/linux/expand-disks

But sudo unmount /dev/sda1 didn’t do the job as it said “umount: /: target is busy”. So I’ve unmounted the disk with the sudo umount -l /dev/sda1 command. Since then I can’t even do sudo in terminal and df -H is not working anymore. It throws “df: cannot read table of mounted file systems: No such file or directory”.

When I type sudo, it says “sudo: effective uid is not 0, is /usr/bin/sudo on a file system with the ‘nosuid’ option set or an NFS file system without root privileges?”

How can I fix this issue please help. Thanks in advance.