Unable to resize last partition on external drive
In Disk Utility a 2 TB disk with three partitions (2x500 GB &amp; 1x1 TB) looks like this:



In Terminal and using gpt you will get the following output:

sudo gpt -r show disk3
       start        size  index  contents
           0           1         PMBR
           1           1         Pri GPT header
           2          32         Pri GPT table
          34           6         
          40      409600      1  GPT part - C12A7328-F81F-11D2-BA4B-00A0C93EC93B
      409640   976562504      2  GPT part - 48465300-0000-11AA-AA11-00306543ECAC
   976972144      262144         
   977234288   976562504      3  GPT part - 48465300-0000-11AA-AA11-00306543ECAC
  1953796792      262144         
  1954058936  1952044320      4  GPT part - 48465300-0000-11AA-AA11-00306543ECAC
  3906103256      262151         
  3906365407          32         Sec GPT table
  3906365439           1         Sec GPT header


The partition with the index number 2 is Volume1, index number 3 is Volume2 and finally index number 4 is Volume3. The 262144-block areas of unallocated disk space are standard Apple partitioning policy.

After resizing the second partition to 250 GB Disk Utility shows the following:



and gpt ...

sudo gpt -r show disk3
       start        size  index  contents
           0           1         PMBR
           1           1         Pri GPT header
           2          32         Pri GPT table
          34           6         
          40      409600      1  GPT part - C12A7328-F81F-11D2-BA4B-00A0C93EC93B
      409640   976562504      2  GPT part - 48465300-0000-11AA-AA11-00306543ECAC
   976972144      262144         
   977234288   488281248      3  GPT part - 48465300-0000-11AA-AA11-00306543ECAC
  1465515536   488543400         
  1954058936  1952044320      4  GPT part - 48465300-0000-11AA-AA11-00306543ECAC
  3906103256      262151         
  3906365407          32         Sec GPT table
  3906365439           1         Sec GPT header


Expanding the fourth partition (Volume3) would require to move its first block (1954058936) to lower block numbers e.g. block 1465777680.

This can't be done with on-board tools like Disk Utility or command line tools like diskutil. 3rd party tools like iPartition or Stellar Partition Manager are capable of doing this though.