MacBook - Permissions problem on external hard drive

I have a 700 GB external hard-drive, a seagate freeagent pro, and I'm having trouble with permissions on it recently. Every time I try to delete something from it it says

"The item "[name of file]" will be deleted immediately. Are you sure you want to continue?"
And when I select delete it says
"The operation cannot be completed because you do not have sufficient priveleges for some of the items"

So therefore I can't delete anything from it.
Also, whenever I copy something from it, a warning comes up...

"You may need to enter the name and password for an administrator on this computer to change the item named "[Name of file]""

...and sure enough, everytime I do anything to one of those files it asks me for the password. Anybody have any idea how to solve this? I've tried looking through various mac forums but can't find the solution. I've looked into repairing the permissions on the disk using Disk Utility, but the "Repair Disk Permissions" button is shaded so I can't click on it.