FWIW: I'm copying some files to our Black Armor NAS this evening and my wife thought that the noise was coming from the air conditioner. In case you don't know, the Black Armor NAS is made by seagate and contains seagate LP series drives. They put the noise in noisy, I can hear them across the room. I have to listen carefully to hear hitachis sitting next to me that are providing the data being written to the seagates.

I think that I'm going to dedicate an ambient track to them...with a video...of my SPL meter.

most name brand externals are crappy i will give you that reagrdless of whos drives are in there.. and yes they for some reason tend to be noisy.
most of the time the drives in there are not the normal drives.

best to buy an empty drive enclosure and add your own drives..
even some higher end units (sonnet) are no better.
they offer drives in raid 5,6 and dont even use Enterprise drives. a known must do for raid 3,5,6 etc.