Disk Encryption not recognising (GPT) 4TB disk

Symantec Disk Encyption refuses to see my 4tb external HDD.  Maybe its a coincidence, but its GPT rather than MBT because of its size.  Windows otherwise sees the disk as normal.  This is both in the Desktop GUI and with pgpwde --enum.  Another (1 TB) USB disk is recognised, as are some internal drives, but they are all MBT.

pgpwde --version shows "Symantec Drive Encryption, Version 10.3.2 (Build 15495)"

Ideas?