Can't unmount a loop backed file but there's no open files?

In your question, you wrote grep pathofimagefile. Have you tried with grep pathofmountpoint?

Also verify that no process running on your machine has your mount point (or a subdirectory of it) set as its current working directory.

sudo ls -l /proc/*/cwd | grep pathofmountpoint will give you those process numbers.

I believe this is what fuser is for. Specifically, fuser -km /path/to/mount/point - note that the -k flag kills processes with files open on this filesystem. You can omit this flag to see a list first.

Wow, this is really old, but to benefit those finding this in the future, here is what I found -- I had nested mounts. That is, I mounted a root filesystem image with a loopback device on /mnt. Under that mount point I had then mounted proc and sysfs filesystems mounted under /mnt/proc and /mnt/sys. Later I had forgotten about the proc and sysfs filesystems when trying to umount the filesystem image.

# mount -o loop rootfs_disk.img /mnt
# mount proc /mnt/proc -t proc
# mount sysfs /mnt/sys -t sysfs
# # ... ages pass
# umount rootfs_disk.img
umount: /mnt: device is busy.
# umount /mnt
umount: /mnt: device is busy.

-- Noah Spurrier