Hardware :: External Hard Drive Placement - Horizontal Or Vertical

I've got seven external hard drives, ranging from portables to full-size, 120gb to 1TB.

Some came with stands (for vertical placement), others did not. some are advertised as vertically standing units (such as the WD MyBook), and some aren't.

So what's the best position? Vertical or horizontal? The obvious answer might be to simply follow guidelines and place vertically the ones that come with stands and horizontally the ones that have feet on one side or which don't have stands. But I'd like to make sure, anyway.

For example, my Lacie 500gb has feet on one side. I placed it vertically and it was good for 4 months or so. Then it began to make intermittent (and loud) clicking noises. I placed it horizontally and all is well now. Been fine ever since.