(chroot) root@ubuntu:/# update-grub
/usr/sbin/grub-probe: error: cannot find a device for / (is /dev mounted?).
(chroot) root@ubuntu:/# ls /dev
agpgart  audioctl  dsp2  kmem	loop4	mem	midi03	mixer1	    null  ram0	 ram13	ram3  ram8    rmidi2	 smpte1   stdin   tty2	tty7
audio	 console   dsp3  loop0	loop5	midi0	midi1	mixer2	    port  ram1	 ram14	ram4  ram9    rmidi3	 smpte2   stdout  tty3	tty8
audio1	 core	   fd	 loop1	loop6	midi00	midi2	mixer3	    ptmx  ram10  ram15	ram5  random  sequencer  smpte3   tty	  tty4	tty9
audio2	 dsp	   full  loop2	loop7	midi01	midi3	mpu401data  pts   ram11  ram16	ram6  rmidi0  shm	 sndstat  tty0	  tty5	urandom
audio3	 dsp1	   fuse  loop3	mapper	midi02	mixer	mpu401stat  ram   ram12  ram2	ram7  rmidi1  smpte0	 stderr   tty1	  tty6	zero