Permission Refused to read my external hard drive

I can't access my WD external hard drive anymore (firewire).
I have an Imac with leopard.
Two weeks ago, my system wouldn't boot anymore. Nothing worked of the classical solutions (safe mode, etc...), so i decided do restore the system from Leopard CD. It created a user called 'previous system' and kept all my datas. The only problem is that now when i try to acces my hard drive, it says that i don't have the permissions to do so (the drive appears on the desktop with a little lock).
 When I try to change the permissions, it doesn't let me do so, even though i have the administrator's rights (unelss i mistake). Safe Mode doesn't help...The otpion 'repair permissions' isnt available in the disk utilities.. I am clueless!! Thanks for your help
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rotemwebAuthor Commented:
Excellent! This post completely solved my problem. I couldn't find it myself.

For info: (in case the link becomes broken)  I executed this command in the terminal:

ls -alO /Volumes     # Use this command for Leopard   <--- or delete this line or comment
ls -alo /Volumes    # Use this command for Tiger or previous   <--- or delete this line or comment
diskutil list
echo "done"

The line concerning my volume showed:

drw-rw-r-T+ 18 agentggd  staff  uchg    680 May 28 08:56 rotemdrive ext

The "uchg" means that the user immutable flag is set
To undo this, I typed:  (don't forget to esape a space in the volume name with \)

chflags nouchg /Volumes/rotemdrive\ ext
sudo chmod 1775 /Volumes/rotemdrive\ ext
echo "done"
It asks for the password (doesn't work with password set to nothing -->must create a password)

And then you can change the permissions in the Volume Infos
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