It will probably stop working. Aerodynamics are crucial to the way a hard disk works. One cannot not simply increase the density of the gas inside the drive by 10x and expect it to continue working.

The helium reduces resistance which results in lower vibration of the read/write heads, which in turn allows for reduced space between the platters and the heads, which allows for a reduced space between the platters and more platters per drive. If the same drive was filled with air rather than helium, the increased vibration of the heads would cause them to crash into the platters - which would scrub the disk clean (which would be bad).

In a vacuum, the head would crash into the platter. The gas provides a cushion. Flying height these days is something like 3nm (yes, you read that right), so the tiniest vibration could cause a head crash in a vacuum.

The heads need a gas of some sort to float on.