Slighty off board sorry. But you guys here will be having the same issues.

I am building a new shelving system to store external HDs (11 so far) (backed up offsite)

This is a raw file picture dump - different from my server where I keep jpg and tiffs for quick access

Can I store/use them vertically ie like books on a shelf

or do they need to be flat so the discs dont fight gravity when spinning (like a record on a deck doesnt)

SMM

Not that I think it matters one iota, Sam, but I think placing them vertically places less stress on the discs. Think of the record: under gravity, holding it horizontal it bends under it's own weight. The discs in the hard drives are very light, but they will tend to bend in the same way. Vertically this wouldn't be the case. When spinning, they are not 'fighting gravity' because the weight is distributed evenly around the disc. My HDD's are all stored and used vertically.

'Ye cannae fight the laws of Phasics, Cap'n' as I read quoted recently
Jon

Well a record deck doesnt work when it on its side

All my computers, my NAS and lappy all keep the disks 'record like'

Thought there might be a reason

SMM

It should not affect the drives being vertical. Every iMac G5 has them in that orientation with more modern computers that are a one unit design doing the same. Nearly every external drive made is vertical with fewer being horizontal. The Apple Xserve raid had 14 drives on their sides. I wouldn't compare modern drives to a record player.

Cool then I can stack them more efficiently in my rack

(I wans only using the record player analogy to try and verbalise the difference between 'horizontal and vertical' not because I think there are technological simiarities)

SMM

slightly OT, but I got a very nice Cavalry 2Tb array from Buy.com for $395 shipped last week - its 2x1Tb disks in a single external enclosure, with fan for cooler disks.
thats a pretty good price considering what 1Tb disks are stand-alone.

Shows up as 1.8+Tb on my workstation, and is very useful for major backups.

also got a 1Tb Cavalry one for my Raw's. that has no fan in it, so- quieter but hotter.

with those 2 around, I've retired all the others.
still got triple redundancy on my Raid array anyways.