Hello, and thank you:
here is my df:
Filesystem 1K-blocks Used Available Use% Mounted on
udev 16402132 0 16402132 0% /dev
tmpfs 3293136 2148 3290988 1% /run
/dev/sdb3 960190496 7220436 904125312 1% /
tmpfs 16465676 8380 16457296 1% /dev/shm
tmpfs 5120 4 5116 1% /run/lock
tmpfs 16465676 0 16465676 0% /sys/fs/cgroup
/dev/sdb1 89111 78300 4074 96% /boot
/dev/sdb2 95767 4706 91061 5% /boot/efi
tmpfs 3293132 68 3293064 1% /run/user/1000
/dev/loop0 89088 89088 0 100% /snap/core/4917
/dev/loop1 35584 35584 0 100% /snap/gtk-common-themes/319
/dev/loop2 144384 144384 0 100% /snap/gnome-3-26-1604/70
/dev/loop3 2432 2432 0 100% /snap/gnome-calculator/180
/dev/loop4 13312 13312 0 100% /snap/gnome-characters/103
/dev/loop5 14848 14848 0 100% /snap/gnome-logs/37
/dev/loop6 3840 3840 0 100% /snap/gnome-system-monitor/51
/dev/loop7 5120 5120 0 100% /snap/canonical-livepatch/41
/dev/loop8 146944 146944 0 100% /snap/chromium/383
I am unable to install virtualbox to /dev/sdb
what might be the reason?