You need to tell me if the boot disk is DOS or WinPE. Both can be created using GSS 2.51
DOS does not support USB so that would explain your problem if you are using DOS.
WinPE will support USB-2 but will not support USB-3 unless you load the correct Vista 32 bit drivers. There may also be an issue if your external drive is USB-3. Some early Western Digital units also provided a driver for the drive itself which needed to be loaded if using a USB-3 port.
So to reduce the number of potential issues, stick with USB-2 ports only and make sure you are using a USB-2 drive. Bear in mind that GSS 2.51 is a very old product, so even the WinPE boot disk is based on a very old version of Windows - specifically Vista 32 bit.
The new GSS 3.1 supports the latest versions of WinPE and currently ships with WinPE based on the Windows 8.1 kernel, for which drivers should be universally available. Some users are using WinPE from Windows 10, and again, most vendors now provide Windows 10 drivers as well.