I use w Thermaltake eSata external docks…
Or search NewEgg for other external docks. I also have a USB only Thermaltake Dock (my first one). eSata probably has little benefit with the USB/FW Drobo but it’s handy for other things and I thought worth a few extra bucks.
I started with 4x1TB drives in my Drobo, and have steadily upgraded. As a result, I have a growing pile of 1TB drives.
I organize my Drobo volumes with top level folders that never contain more than 900GB of data (whatever I feel comfortable packing into those 1TB backup drives). You can pick whatever size you want, based on the size of your backup drives. If I had non-vital data I would just segregate that into a dedicated top level folder that didn’t get backed up.
I use SecondCopy to back up the Drobo top level folders to the backup drives.
SecondCopy does changes only and will delete files on teh target not on the source… very flexible and whatever you want in terms of backup policy. The only thing I do not like about SecondCopy is that they do not yet have a services based version that works with 64 bit Win OS’s. That means I have to leave my file server (which usually has the Drobo attached) logged in.
The nice thing about the eSata docks is that you only need one or two, and then can you buy raw drives without paying for a case. Of course, drives with simple USB enclosures can be had for the price of a raw drive but they tend to come with a 1 year wearranty that is voided if you want to pull the drive out (I have a few WD Mybooks from before I got smart and found the external docks).
You could also get a Sata backplane box and eSata port multiplier card to plug into existing empty 5.25" slot(s) if you have them, for easy all in one hot swap, or an external multi-bay enclosure.
The above is the method I use for my “offline” copies that rotate off site.
For daily backups I have 2x2TB drives in the Win file server with the Drobo. SecondCopy runs automatically every evening, ensuring I have one full up to date copy. I rotate two offline copies.
I thought about another Drobo or a Raid box as suggested above. I decided that my backups should be as technically simple as possible. That is the Hail Mary Pass and there I feel better knowing I can pop those drives into any Win box and read them. No Raid issues, etc. Plus, I have noticed over the years that bad luck and disasters happen in small groups. That’s why I keep 4 copies of my important data and keep the backups as simple JBODs. If my Drobo array crashes I don’t want to worry about doping something stupid to the Raid box backup in the heat of the restore battle, or just have a really bad day lose that array too.
Plus, reading a lot of NewEgg reviews of cheap Raid boxes, I get the feeling that more Raid arrays are lost due to software problems or other issues beyond hard drive failure than are saved by Raid 5 redundancy.
A lot of the above is just religious and philosophical. Do whatever makes you feel comfortable.