Error trying to partition external drive

I have an external 500GB HD that I use as a Time Machine backup for my rMBP. I had partitioned it into 2 a while ago; intending to store large files on the second partition but have now changed my mind. I deleted the second partition and am now trying to re-size the Time Machine partition to the full size of the drive. It works at it for a while and then gives me this error:
View attachment 575727
What can I do to resolve this? Thanks!