i struggled a bit getting backups going for our (mostly mac) office at doctr.com, but now things are finally up and running. i combined a first gen drobo, an airport extreme (wired over gigabit), time machine, and multiple macs with os 10.5.6 (leopard).

i mostly followed the “use sparse image” section from this post. here is what i did step by step:

1. setup drobo with drobo dashboard directly plugged into one computer, format for max size (i chose 8TB because it would be faster boot time than 16TB, and frankly right now only 2TB drives are at a good price point, which makes 5.5 available TB if you have 4 x 2TB drives. i currently have 4 x 1TB drives in the drobo which makes 2.7TB available space. you can estimate available space with the drobolator), even if you don’t have that much space, this means you can slot in some bigger drives in the future without touching your setup, so make this max size BIG.
2. next format the drive with os x’s disk utility with “Mac OS Extended (Journaled)”. if you don’t, you won’t be able to use it for time machine over the airport extreme. disk utility should see it as an 8TB drive (even though it really is much smaller). this step is currently incorrect on this page. you must chose “Mac OS Extended (Journaled)” (aka hfs+). this setup will absolutely not work if you choose just “Mac OS Extended”.
3. the next step is the step outlined here under the heading “Best Method: Use a Sparse Image”. i repeated this step for all our macs at the office, making a sparse image for each with the correct hostname of each machine, the correct MAC address for each machine, and a different name for each -volname. for the sizes, i took the total disk space of each machine and added a bit of headroom for extra weekly backups with time machine. once time machine uses up the space alloted by the sparse image for that particular machine, it will start deleting older weekly backups. currently we don’t have any machines that are even using their full disk capacity, so i have a feeling my headroom will be plenty. make sure that when you add up all the sizes of your sparse images, that your total is less than 95 percent of the drobo available space you estimated with the drobolator. so i, for example, have 4 x 1 TB drives in the drobo, which gives me 2.7TB of available space, so i should keep the sum of all my sparse image files under 2.565 TB. the nice thing about definining these sparse image files is that you can make them bigger later, if, for example, you add a new drive into a machine (or if you upgrade the total usable disk space of your drobo by putting more/bigger drives into your drobo). to increase the sparse image size , “disconnect all users” in the airport utility and plug your drobo directly back into a tower or laptop.  in the command line:
   $ hdiutil resize -size 1500g nameofsparseimage
   you can also add more sparse images by doing the same thing, disconnect all users, plug the drobo back into a machine, add a new sparse image for a new machine you want to backup.
4. once you have all the sparse images on the drive, eject it from your computer, and plug it into the airport extreme (via usb). now setup your airport extreme with the airport utility. choose “Manual Setup” and the “Disks” tab up top. the airport extreme will not report the right size of the drive, don’t worry about that. under the “File Sharing” tab, check “Enable File Sharing”. for me, the way it seemed to be happiest was to choose “Secure Shared Disks: With a disk password” and “Airport Disks Guest Access: Not allowed” and leave “Advertise disks globally using Bonjour” unchecked. Click the “Update” button at the bottom of the airport utility and let the airport extreme reboot completely. i had some problems when playing with configs that the airport extreme wouldn’t reboot properly. a friend noted that this was because the airport extreme sometimes pulls power from the connected disks and doesn’t totally reboot itself. my solution was to unplug the usb from the drive and unplug the power cable from the airport extreme. then repower the airport extreme, then plug in the drive once it has booted back up again.
5. now you can configure time machine. on one of the machines for which you have made a custom sparse bundle, open a finder window and select your airport extreme from the “shared” tab on the left. click the “Connect” button and enter the disk password you defined in the airport utility. the drobo should now be mounted as a shared drive in your finder window, and you should be able to see the sparse bundles you previously made there. now open time machine preferences on this machine and choose disk/change disk. if you have followed everything above, you should see the drobo in the list of available drives. you won’t see any different partitions or the sparse bundles, just choose the drobo drive. time machine may ask you for the disk password again, this can be your username and the disk password you set in the airport utility. (backups should work for all users on the same machine, even if you don’t expressly go into each user and setup time machine, this should be a system wide backup.) now start your backup. when your backup is going, the sparse bundle should mount as a drive on your machine (maybe even on your desktop if you have the Finder pref “Show these items on the Desktop:” “External Disks” selected.) your backups should be only written to this sparse bundle and not write anything extra to the drobo. verify this by navigating in your finder to the airport extreme and make sure that only your sparse bundles that you first made there are listed. if there is anything extra apart from your original sparse bundles, you made a mistake in step 3 with the name of the machine or the mac address. each machine should only write to its own sparse bundle for which you have defined a set size. now time machine won’t try to eat up your entire drobo’s space (in my case 8TB) and stay within the size parameters you set in step 3.

this setup has been up and running for a few days at this point, and i can use time machine (though it is a bit slow) on each machine to step back in time. i will report later if there are any issues with the airport extreme requiring reboot or issues with the drobo. for now it’s all crunching along quite nicely, and i’ve even swapped some disks in and out of the drobo to upgrade my total raid space. i don’t have an offsite backup solution yet, but i’m considering doing backups every two weeks using super duper! with external terabyte drives for each machine. all in all, the drobo is an awesome raid solution that works so painlessly with different sized drives, but getting it to work with a multi-mac setup was a tad more than i bargained for. feel free to leave comments if i left something out, or if something in this outline doesn’t work for you.