Well...my experience is that there is no harm in doing so...I've done it for a number of years due to my lazy nature and the fact that I fool around a lot with drives and my two desktops. And I haven't put the side door on my cases in years...because I like to fiddle with components and I'm lazy. As for dust concerns...I routinely use canned air and a small brush to minimize/neutralize any possible overheating due to dust/debris (I don't have pets and my computers are in a apartment building).
I've had them flat...standing on the small/narrow end...leaning against the case...at no time did I perceive any problem with whatever arrangement I chose to employ. It's only during the last year that I decided to put them all inside the bays built for them .
I initially started doing this because I had a number of smaller drive (1GB or less) and a lot of items I wanted duplicates of, backups, and movies. The advent of 2TB drives and larger has convinced me to just set my smaller drives aside.
The one thing which I suppose could happen...is loosening of the connections to the drive, depending on what type of connectors you use.
Laying the drives flat on a hard surface would possibly lead to overheating of the drive, if done for a protracted period.
Edited by hamluis, 11 April 2015 - 04:21 PM.