external hard drive won't erase and now is not recognized

My 1 TB WD external hard drive was partitioned into 2 partitions. The top partition was for Time Machine, the bottom for my personal files. Those personal files are not on my regular hard drive so I decided I needed to get another external hard drive to back them up. I bought a 2 TB WB and set up Time Machine with that drive to back up my computers hard drive as well as my 1 TB external hard drive. Through my online research I discovered I couldn't make the 1 TB into one partion without deleting my personal files, so I figured since it was backed up on the 2 TB it would be okay. Just to be safe I backed it up onto a 500 GB Seagate as well (this drive crashed a while ago and then randomly started working again, so not too trustworthy - if anyone knows how I can make it trustworthy again, that would be fantastic). Then I partioned the 1 TB into 1 partition. Success. I went to erase it and selected Zero Out Data. This failed "Volume Erase failed with the error: Disk object invalid or inable to serialize." And then my hard drive disappeared. I've been able to open it in Disk Utility and partion it again into 1 partition, but each time I try to erase it, it always fails. Is something wrong with my drive? Is this external hard drive truthworthy to hold important files without crashing again?

Here's a screenshot of what happens when it fails. Yay.

After the Zeroing out doesn't work, this box pops up. And then my drive disappears from the desktop.

At one point the partition failed, but it seems to have gotten over that. Although each time I try to erase and it fails, I have to rename and repartition my 1 TB external hard drive so it shows up on the desktop again.