I have been shooting digital now for nine years. In that time I have a growing collection hard drives.
From a fairly early stage I standardised on LaClie Rugged Firewire drives, ranging from 160 GB to 1 TB, as up to now I have always had Mac Laptops.
I also have three mains powered desktop drives, A old 1 TB LaClie which I now keep my best photographs on as DNG format. A Drobo Array used as a backup target, and my new drive which is a G-Tech EV Thunderbold2 with two 1 TB drives configured at RAID0.
I still have a few things to consider and thats where I put things on the new machine. At the moment my main Lightroom Library is on the internal storage and I have placed all the photographs on the fast Thunderbolt2 external array, and have a 1 TB USB3 drive split into two partitions, one as a Time Machine target for backup and the other holding my iTunes Library.
My oldest LaClie drive is also attached and is set as a Lightroom import backup target and also as a catalogue backup target.
With just the G-Tech and USB drive my office is now silent and I can hear the birds out side.
The Drobo is quite noisy and but gets used once a month for backups.
Interestedly before I bought an extra Thunderbolt2 to Firewire converter and the new Thunderbolt drive; I connected directly into the Drobo and daisy chained everything off that. It definitely slowed things down and I also thought I had a finder issue and spent sometime trying to diagnose the issue.
On boot up my finder would hang for about two minutes before I could use it. It turned out only to do this when the Drobo was connected, so I do wonder if the Drobo is ready for replacement.
A couple larger cheap USB drives also have a copy of all my photographs on, one of these I keep in the office in Lincoln and the other at home, they get swapped and updated monthly so if the worst should happen I still have my work.
Cloud storage also plays its part. I tend to use Dropbox as its available on all the platforms I use from Linux, Mac and Window, plus my phone. I stick a copy of my Lightroom library here.