My passport Ultra read only on mac (can't write or delete)

I have a My Passport Ultra portable external disk which I use with both PC and Mac. I have a mac with which this disk is working just fine both for reading and writing, and it also works with the PC. Now I am trying to use a third computer, also a mac, which has Yosemite installed. I can’t write anything nor delete anything in the drive. I can only read. Please help!!!

Hello, what format do you have on the external drive? Have you tried performing a disk check on first aid?