External USB hard drive and risk of internal condensation?

Condensation is a real danger for hard disks.

You can see in a real-life youtube demonstration of a data-recovery specialist, what a hard disk looks like when taken out of the freezer and briefly turned on, just full of scratches :

scratched disk

Such scratches could possibly damage the disk to a point that even a data-recovery specialist will be unable to recover the data.

A Control Data (later Seagate) factory packaging manual for hard disks says:

If you have just received or removed this unit from a climate with temperatures at or below 50°F (10°C) do not open this container until the following conditions are met, otherwise condensation could occur and damage to the device and/or media may result. Place this package in the operating environment for the time duration according to the temperature chart.

acclimatizing times

It seems that dangerously low temperatures start when the computer is brought from below 50°F (10°C) into room-temperature, and it may need several hours for acclimatization. This long time is explained by the fact that in a mechanical disk, the head is supported by airflow entering through special air-intakes. These intakes are heavily filtered against dust, but not against humidity. They are also small enough, which slows the evaporation process of internal humidity.

You could possibly minimize the acclimatization time by wrapping the disk in watertight plastic while it is acclimatizing, to reduce the humidity that would enter via the air-intakes. You should allow for some drying-off time after unwrapping the disk, for the humidity in the air already contained inside the disk.

This is not the only danger, as explained by data-recovery specialist ReWave Recovery :

A hard drive is at risk for sudden temperature changes including overheating and condensation.

A sudden change in temperature that causes condensation inside the hard drive can cause the material on the platter to evaporate which causes the read/write heads to stick to the platter and stop it from rotating.

Overheating can also be an issue. Overheating can cause the platters to expand which makes the read/write heads travel farther to read the data. The expansion of platters can cause friction which can lead to a head crash.

I checked PDF on Seagate(can't find much info on WD), and find some interesting info. Its for laptop HDD, the one inside portable external USB drive.

According to the PDF(1, 2), the hard drive can operate at 0-60C and a relative humidity of 5-90%. However Seagate also limits the gradient, the temperature gradient is 20 or 30C/h, humidity is 30%/h.

So according to these PDF, if your HDD is Seagate, temperature wise, it should be safe, for humidity, it varies from 58% to 90%, considering its the extreme case and just a little over 30% I wouldn't worry much then.