Can't unmount USB volume

[Laurent CHARLES]

Hello,

I'm facing a problem with my newly installed warty on a laptop HP zd7000 
(zd7320ea). I can't unmount USB volume, neither from the gnome desktop, 
nor from the command line.
  % sudo umount /media/sda1
  Password:
  umount: /media/sda1: device is busy
  umount: /media/sda1: device is busy
  %

And when I try to get further information from the system...
  % fuser -m -u /media/sda1
  %
  % lsof | grep sda1
  %
...nothing
The device is mounted and contains data
  % ls /media/sda1
  img_1631.jpg  img_1656.jpg  img_1675.jpg
  %

Did anyone face the same problem (and solve it)?

Thank you for your help.

Regards

-- Laurent