There are GPTs at both the beginning and end of the disk.
It is insufficient to clear only the first one. 

Calculate number of sectors like this:

fdisk -l /dev/sdb
Disk /dev/sdb: 2000.3 GB, 2000398934016 bytes
64 heads, 32 sectors/track, 1907729 cylinders
Units = cylinders of 2048 * 512 = 1048576 bytes
Disk identifier: 0x00000000

Number of sectors = 64x32x1907729 = 3907028992

The following pair of dd commands ultimately worked. I had actually already tried something similar but the dd command I used for the end of the disk was somewhat different and failed to work for reasons unknown. These commands worked:

# dd if=/dev/zero of=/dev/sdb bs=512 count=2
# dd if=/dev/zero of=/dev/sdb bs=512 seek=3907028992