I don't know whether this is problem of Mandrake 10 or Konqueror.
I cannot unmount hard disk partitions(vfat, ntfs), network drive via samba, and usb memory stick after I access those partitions or drive via Konqueror. If I mount the partitions in shell and access ONLY in command window then I can unmount the partitions.
I tested it without supermount on kernel 2.6.7 and with supermount on kernel 2.6.3-13mdk version.

Does anybody know what could cause this problem ?

I have a the same problem; keep getting a device busy message when I try to unmount after doing things in koqueror.The way I get around it is to kill the processes accessing the device/partition/share using fuser:

# fuser -km <mount point>

Then I can umount the device/partition/share. You can see what process is tying up things by running:

# fuser -vm <mount point>

I've always found it's a process called "fam", short for file alteration monitor( see man fam for full explanation).

I guess using 'fuser' is the solution for the time being.
Thanks for the tip again.

I think I figured it out. I think it's a problem/bug with konqueror as it interacts with fam. If you go into kde control center>System>KDE Performance, you'll see a section called "Preloading". Under "maximum number of instances kept preloaded" change it from the default 1 to 0. I did that and haven't had a lockup since. My guess is that fam keeps running while the preloaded instance of konqueror is present in order to keep konqueror updated as to any file alterations and that's what's locking things up. If you remove the preloaded konqueror by setting the preload to 0, no more lockups