terrible write performance when almost full

StephenB wrote:
If so, you could try attaching silica gel packs to the unused drive tray. I suspect a lot of the damage is due to condensation, the gel packs might reduce or prevent that. Also, when the unit is running the temp is probably high enough to prevent most condensation, so if you are attached to external power you might keep the unit running.

I'd ignore the silcagel. It's going to suck up all the water it can hold in just a few hours, days at most, if the humidity is high. After that, it's just useless. Leaving the units powerd on is a good idea tho. Even if there is a salt buildup, it's not as corrosive dry, as it is when it's collecting condensation.

Using a old refrigerator isn't cost effective. Worse yet, in the case of an air-conditioned room, the heat from the refrigerating unit is dumped in the room, adding to what has to be dealt with already.
A sealed cabinet, the only thing that can get degraded by moist air, and salts, are power and USB connectors. They can be replaced, either by replacing the whole cabinet, or the contacts them self, if need be. That should be the easy solution.
B!