System is corrupted installing packages

[debuser <debuser@yandex.ru>]

Hi!

I have the flash card image which contains a lenny dustr and I'd like to
upgrade the OS there up to sid.

I mount the image to the dir 'flash':
mount -o loop,offset=16384 -t ext2 q.raw flash

and I chroot into it:
LC_ALL=C chroot flash/ /bin/bash

Everything is ok. Then I run 'nano /etc/apt/source.list' and replace
'lenny' to 'sid'. 

After I run 'aptitude update'. Everything is still ok.
Then I run 'aptitude upgrade -y', all necessary packages are started
downloading. When the process is finished (system downloaded and
configured all it had needed) I exit from the chroot.

But, when I try to umount dir 'flash' I get:

#umount flash/
umount: /home/debuser/work/flash: device is busy.
        (In some cases useful info about processes that use
         the device is found by lsof(8) or fuser(1))

I do and get:
#fuser -m /home/debuser/work/flash
/home/debuser/work/flash:  2832rce  8811rce

if I do:
#fuser -m -k /home/debuser/work/flash

it ends these processes and my ssh session is terminated. After that I
can't log in to the server via ssh.

I try using 'fakeroot' but I was not succesfull.

Is there any way to install and configure packages on mounted image
without following results? (without these troubles of some processes and
it's terminations )