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> tape drive - http://web6.scan.co.uk/Products/Info.asp?WPID=14022
> (thanks for the link, Martin) - by my calculations provides more
> storage at only slightly slower speeds at half the price. Is there any
> overriding reason why I should go with the more expensive and
> lower-capacity DDS-4 drive other than speed?

The ADR (Advanced Digital Recording) tape format is an interesting
option, one I hadn't seen much of, before.  Look as if OnStream (spun
off from Philips Electronics in 1998) invented and rather tightly
control the format, though other companies such as Verbatim do make
compatible media under licence.

If you're reasonably serious about backup, you need to plan for a
rotational backup scheme and retirement of tapes as they become worn
(and well before they start failing).  That requires at least a couple
of dozen tapes for the first couple of years.  Therefore, cost and
availability of media are (or should be) a major factor.  Also, tape
heads wear out and need to be replaced (more rapidly for helical-scan
systems -- which ADR turns out _not_ to be), and somehow that always
ends up being cheaper for tape technologies in which there is heavy
competition.

The less-tangible consideration that comes most immediately to mind is
that, if one's OnStream ADR drive fails, the only possible replacement
that would suffice to restore your accumulated backup sets is another 
OnStream ADR drive.  In that sense, you're somewhat locked in:  I'm 
not seeing ADR drives from anyone else (though I might have missed
them).  

If OnStream are smart, they've priced the drives to attract people into
the system, and are making up the shortfall on media cost after they 
get people to buy in.   I see the following prices for mail order 
(http://www.finnsoft.com/priclist/verbat.htm) of Verbatim-brand tapes:

42 euros for 30 GB ADR tape
33 euros for 30 GB ADR tape, four-pack.
66 euros for 50 GB ADR tape.
44 euros for 50 GB ADR tape, four-pack.
47 euros for ADR cleaning tape.

(You don't get much lower pricing than Verbatim, without going no-name.)

Capacities are usually quoted on the basis of a nominal 2:1 compression
ratio.  The PDF datasheet for Onstream's "ADR2" series says this is a
linear-serpentine recording technology (a good thing) like DLT, SDLT,
and LTO, rather than a helical-scan method typified by DDS/DAT, AIT, and
8mm (which puts heavy wear on tapes on heads).

I'm not sure what software other than NovaStor's TapeCopy and Yosemite
Technology's Tapeware will support OnStream's drives.  My recollection
is that BRU will do it.

On balance, I'd personally _still_ go with a DDS4 drive using either DDS3
or DDS4 media, mostly because once you've survived life with one
oddball, less-standard tape format, you're not in a hurry to repeat the
experience.  But ADR does seem at a quick glance to be well designed,
and might be a good bet after all.  As they say in the gaming industry,
"Ya pays your money, and ya takes your chances."

-- 
"Is it not the beauty of an asynchronous form of discussion that one can go and 
make cups of tea, floss the cat, fluff the geraniums, open the kitchen window 
and scream out it with operatic force, volume, and decorum, and then return to 
the vexed glowing letters calmer of mind and soul?" -- The Cube, forum3000.org

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