Error trying to partition external drive

[Falcon9]

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:

What can I do to resolve this? Thanks!

 

[Weaselboy]

Try gong back to the First Aid tab and doing a repair disk.

If that does not work, I think the problem is you are using the wrong partitioning scheme there. Normally you would want the OS X default GUID partition table. To fix that you will need to reformat the entire drive.