external hard drive won't 'write' on desktop but does on laptop

I have a free agent drive that is readable and write-able on my mac laptop. However when I connect to my imac desktop it will only read - info says READ ONLY. HOw can I get it to write? I don't want to erase any of the stuff I have on this hard drive - both are OS X Lion. HELP! Thanks all!

PS it will not allow me to change permissions on my desktop.

I'm going to guess.. that the drive is NTFS, and you have a third party application installed that allows you to write on the laptop.

yes it's NTFS - perhaps what you say explains it. Trying a 3rd party software on the desktop to see if it will enable it.