It's awfully hard to make a watertight package which can hold 4km pressure for 2 years and which uses just o-rings...
4km is about 6000psi, and according to
Wikipedia - Hydraulic Machinery:
Elastomeric seals (O-ring boss and face seal) are the most common types of seals in heavy equipment and are capable of reliably sealing 6000+
psi (40+
MPa) of fluid pressure.
I'm not saying that building one of these memory modules is a simple task, but then again pressures in the 6000psi region are common in hydraulic machinery. I believe the A380 hydraulics operate at 5000psi.
As far as checking if there was water in the module prior to powering it up - would not the simplest way be to weigh it?