need help with external HD "activation" after changing HD on macbook pro

can somebody help me
im not computer "savvy" so i need simple explanation to my problem

i had macbook pro 160GB..i just changed the HD to 500GB..i bought a seagate HDD..i changed this ok and installed operating system/applications..everything ok

however..i used before an external HDD to just transfer my movies/music to keep copy..its a seagate freeagent goflex 500GB..i could just drag the movies to numerous files on the external hdd and it copied them no problem

now..with the new HDD installed the seagate goflex will not work 100%..i can transfer movies to itunes but i cannot add anymore movies/music to the external drive anymore..it says " cannot transfer because this folder cannot be modified"..in other words there is a block on transferring anything to the external HDD

what can i do to once again "activate" the seagate goflex to accept documents

help appreciated if you know..in simple terms..step by step

thanks

I suspect your external drive is formatted as NTFS - OSX can read but not write to NTFS format disks

You probably used to have an NTFS driver installed but you haven't re-installed one since the rebuild.
I'd recommend Paragon NTFS personally. Though there are alternatives: http://www.mac-forums.com/forums/os-x-applications-games/231834-ntfs-program-mac.html#post1199588