luksClose / device is busy

[scar]

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i am working on a friend's laptop, that has an encrypted hard drive.  i
took it out and plugged into my system, then i used these commands to
mount it and work on it:

sudo cryptsetup luksOpen /dev/sde5 sde5_crypt
(this makes available the -root and -swap partitions in /dev/mapper)
sudo mount /dev/mapper/laptop-root /mnt/laptop-root

then when i am finished i want to unmount it:

sudo umount /mnt/laptop-root/
sudo cryptsetup luksClose sde5_crypt

but i get this error: Device sde5_crypt is busy.

i don't see why/how it's busy since i already unmounted the filesystem
on it.  how can i find out what's keeping it busy and stop it?  thanks

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