Items can't be copied because you don't have permission to read them

I have a LaCie Cinema HD which has about 500 Gb of movie files on it. I had it connected to my Airport Extreme by Ethernet. I then have my iMac on a wireless network and could transfer files from my iMac to the LaCie with no problems. I then tried using a wifi dongle on the LaCie rather than the ethernet connection. Again it just worked fine. I was a little worried about security on my iMac as I had set it up straight from the box as a single user, which I since learned was an administrator account. I reinstalled my iMac (after reformatting its internal disk) and set it up as a 2 user system (administrator & standard) as recommended in the magazines. My problem now is that whenever I try to copy files to the LaCie (using either account) I get a window asking me to "type an administrator's name and password to allow Finder to make changes". When I do this I then get another window saying "one or more items can't be copied to (folder on LaCie) because you don't have permission to read them. Do you want to copy the items you are allowed to read?" There are "Stop" & "Continue" buttons. Nothing happens regardless of which button I click. I can copy files from the LaCie to the iMac but not the other way round. LaCie tech support suggest resetting my LaCie (which would wipe the files on it!). I'm reluctant to do that as a) I can't afford to buy another external drive to back 500 Gb of files up first & b) I don't think that solution will make any difference anyway. There seems to be a few people with this problem with external drives but all my web searches seem to relate to Linux & OS 9 systems, or NAS drives from other manufacturers that have eventually issued a patch. I think the problem is with my iMac rather than the external drive. Any ideas anyone - please! I'm on the verge of going back to the dreaded Windows, I'm that desperate!