Steps before obarun-install execution
sda2 = system root ( / )
sda1 = system boot ( /boot )
sdb1 = system home ( /home )
~ % mkdir /mnt/obarun
~ % mount /dev/sda2 /mnt/obarun
~ % mkdir /mnt/obarun/boot
~ % mount /dev/sda1 /mnt/obarun/boot
~ % mkdir /mnt/obarun/home
~ % mount /dev/sdb1 /mnt/obarun/home
The problem
After leaving obarun-install program,
~ % umount /mnt/obarun/home
~ % umount /mnt/obarun/boot
~ % umount /mnt/obarun
~ % umount: /mnt/obarun: device is busy.
~ % cat /proc/mounts
proc /proc proc rw,nosuid,nodev,noexec,relatime 0 0
sys /sys sysfs rw,nosuid,nodev,noexec,relatime 0 0
dev /dev devtmpfs rw,nosuid,relatime,size=6101476k,nr_inodes=1525369,mode=755 0 0
run /run tmpfs rw,nosuid,nodev,relatime,mode=755 0 0
/dev/sdc /run/archiso/bootmnt iso9660 ro,relatime,nojoliet,check=s,map=n,blocksize=2048 0 0
cowspace /run/archiso/cowspace tmpfs rw,relatime,size=8388608k,mode=755 0 0
/dev/loop0 /run/archiso/sfs/airootfs squashfs ro,relatime 0 0
airootfs / overlay rw,relatime,lowerdir=/run/archiso/sfs/airootfs,upperdir=/run/archiso/cowspace/persistent_OBARUN_2018-09/x86_64/upperdir,workdir=/run/archiso/cowspace/persistent_OBARUN_2018-09/x86_64/workdir,index=off 0 0
devpts /dev/pts devpts rw,nosuid,noexec,relatime,gid=5,mode=620,ptmxmode=000 0 0
shm /dev/shm tmpfs rw,nosuid,nodev,relatime 0 0
cgroup /sys/fs/cgroup tmpfs rw,relatime 0 0
cgroup /sys/fs/cgroup/cpuset cgroup rw,relatime,cpuset 0 0
cgroup /sys/fs/cgroup/cpu cgroup rw,relatime,cpu 0 0
cgroup /sys/fs/cgroup/cpuacct cgroup rw,relatime,cpuacct 0 0
cgroup /sys/fs/cgroup/blkio cgroup rw,relatime,blkio 0 0
cgroup /sys/fs/cgroup/memory cgroup rw,relatime,memory 0 0
cgroup /sys/fs/cgroup/freezer cgroup rw,relatime,freezer 0 0
cgroup /sys/fs/cgroup/net_cls cgroup rw,relatime,net_cls 0 0
cgroup /sys/fs/cgroup/perf_event cgroup rw,relatime,perf_event 0 0
cgroup /sys/fs/cgroup/net_prio cgroup rw,relatime,net_prio 0 0
cgroup /sys/fs/cgroup/hugetlb cgroup rw,relatime,hugetlb 0 0
cgroup /sys/fs/cgroup/pids cgroup rw,relatime,pids 0 0
cgroup /sys/fs/cgroup/rdma cgroup rw,relatime,rdma 0 0
/dev/sda2 /mnt/obarun xfs rw,relatime,attr2,inode64,noquota 0 0
airootfs /mnt/obarun/var/cache/pacman/pkg overlay rw,relatime,lowerdir=/run/archiso/sfs/airootfs,upperdir=/run/archiso/cowspace/persistent_OBARUN_2018-09/x86_64/upperdir,workdir=/run/archiso/cowspace/persistent_OBARUN_2018-09/x86_64/workdir,index=off 0 0
airootfs /mnt/obarun/var/cache/pacman/pkg overlay rw,relatime,lowerdir=/run/archiso/sfs/airootfs,upperdir=/run/archiso/cowspace/persistent_OBARUN_2018-09/x86_64/upperdir,workdir=/run/archiso/cowspace/persistent_OBARUN_2018-09/x86_64/workdir,index=off 0 0
How to solve the problem
Umount twice /mnt/obarun/var/cache/pacman/pkg
~ % umount /mnt/obarun/var/cache/pacman/pkg
~ % umount /mnt/obarun/var/cache/pacman/pkg
~ % umount /mnt/obarun
~ %