Can't use my old HDD after upgrading with new one,old is GPT protected

Hi everyone! First of all I would like to thank you in advance!
Recently I upgraded my old HDD on my MacBookPro. I made a clone of the old disk, installed the new one in the MBP and putted the old one in external enclosure. Now I'm trying to use the old disk with the enclosure, but it doesn't appear in Finder, it appears in Disk Tools, but I can not perform any actions on it. I tried to erase it, but with no success! The old disk is GPT protected and I don't know what to do. Tried to erase it with Windows and Linux with no success neither. I know the enclosure works because it recognize every other disk, but my old one is untouchable to my Mac OS! I'm looking for possible ideas on how to make my old HDD usable with the enclosure?