Apricorn Aegis secure flash drive should fit your security needs.  Including validated 256-bit military grade hardware encryption and a 7-15 activated alphanumeric PIN.  Specialized software or drivers are not required and it’s compatible with PCs, MACs, Linux, and embedded systems.

The flash drive instantly locks once unplugged from your computer’s USB port.  Dust and water resistant, with internal components being protected by an epoxy compound which is virtually impossible to remove without damaging the electronics.  The Apricorn Aegis Secure Key comes in 4Gb, 8Gb, 16Gb, or 32Gb models.  The 4Gb model sells for around $65 and the 32Gb model sells for around $180.