Ubuntu 15.10 - "BusyBox built-in shell (initramfs)" on every boot

I've the same issue and I found the solution that worked for me at Initramfs Prompt When Ubuntu Boots.

So, first type:

exit
BusyBox v1.17.1 (Ubuntu 1:1.17.1-10ubuntu1) built-in shell (ash)
Enter 'help' for a list of built-in commands

(initramfs) exit

You'll get something like this:

/dev/sda1: Unexpected …
fsck exited ….
The root filesystem on /dev/sda1 requires manual fsck. 

And after, use the following command:

(initramfs) fsck /dev/sda1

If you are dropped into initramfs, it means that the boot flow stop at the stage of mounting your root filesystem, which is usually in somewhere of your hard disk.

To understand the above statement, you need to know the knowledge about the booting flow, which in general is:

1. BIOS
2. Boot loader (e.g. grub for Ubuntu)
3. Kernel and initial ram file system (initramfs, yeah where you are)
4. root file system (your target system)

The root causes to make you stop at initramfs could vary. I get used to solve this kind of issue by the following steps:

1. Prepare a Ubuntu live USB stick
2. Use this live USB to boot up your PC
3. If 2nd step works (means you could boot into live desktop and begin to use the Ubuntu system), it means your PC is supported by Ubuntu (for basic usage at least)

4. (optional) dump the system and hardware information. use the information to ask for help (for example, paste them on askubuntu.com) Generally please paste the output of these command in a terminal at least:

   lspci -nnvv 
   mount
   dmesg
   dpkg -l
   

5. If 2nd step works, install that workable Ubuntu distribution.

Modify your kernel boot parameter by setting the root=/dev/sdaX option. sdaX would be your / or root partition. Upon booting the next time, you will see that your initramfs tries to mount the partition before trying to access /etc/fstab and mounting the file systems.

See question "Does initramfs use /etc/fstab?" for more details.

Source: unix.stackexchange.com/a/120862/1226639