What exact dock brand/model did you buy? It seems like some reviews say the dock damaged their hard drive.
If I need to switch on the dock (with hard drive) for 24hours do I need a dock with fan?
Rosewill RX301-PU3-35B USB 3.0; December of 2015; 18 dollars; Newegg;
I have no idea how it would damage the drive. You can insert it backwards in which case it might damage the connectors if you tried to force it to fit, but that is user error like using a standard screwdriver on a Philips screw.
You have to read between the lines and discount a lot of stuff you read in online reviews....as you likely know.
Fans and heat:
I have seen complaints about heat on the fully enclosed units that you were originally considering. As well as failures of the included connections and circuit board.
These things aren't exactly the peak of human engineering. That would be the Zippo cigarette lighter or the F4U Chance Vought Corsair. Or the Slinky.
My dock has the drive standing vertically, with the top 2/3 in the open air. I don't leave it on and running for more than a couple of hours at a time while I back up. Other than that, it is in a closet.
If I bought one like you linked earlier in this thread, where the drive is laying down, I'd probably leave the lid open to let heat out. Might help.
I have no idea about the worthiness of any built-in fan on any of these things.
I would encourage you to NOT leave the thing on and running constantly.
You may have decided that is an absolute requirement. I don't know. That may affect reliability and your eventual satisfaction level due to heat. Or not. Roll some dice.
All of which is part of the reason why I use an internal for primary backup and use my dock only for an hour or two every couple of months.