Still, that is approximately what happens.
Partial pressure put simply:
Forget absolute pressure. What matters in leaks is the pressure of each gas individually.
There is 0.00052% of He in the atmosphere. Assuming 1 bar of absolute pressure this means there is a partial pressure of 0.0000052 bar of helium in the atmosphere.
If the drive is filled with 1 bar absolute pure helium the difference will be 1-0.0000052 = 0.9999948 bar. That is the pressure that is important. There is no way the helium will not leak out. There is no such thing as a closed system. The system will also leak air in but far less. The result is a vacuum in the drive. In the end that vacuum will be filled with air but that takes far slower.
Now how long will the loss of helium take?
A 3,5 inch drive is about 0.3l. I assume that half of that is filled with hardware so I assume 0.15l He. A properly welded system without any connectors is probably in the range of 10^-12 mbar*l/s He leak tight. If I assume the drive will work at 0.5 bar helium we can take 10^12*500 mbar *0.3l = 150 * 10^12 seconds.
A year has 3.15 * 10^6 seconds. That is almost 5 million years. Not really something to worry about.
Yes there is a leak. Yes the helium will escape. No it doesn't matter because it just takes extremely long.
With a bad weld the time would drop significantly. However, the detection is easy. Helium leak detectors are commonplace, to detect minute leaks in high purity systems.
Sources:
Dimensions drive: Wikipedia.
Leak rate: I can get connectors to 10^-11 as standard items (Swagelok VCR full metal seal). Fully welded systems are probably better than that.