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The Drobo storage device is a beautiful piece of technology. It is quite possibly the most user-friendly RAID (like) device on the market. With very little effort, you can have 3TB+ of failure-protected storage at your fingertips.

The problem with the Drobo is that in order to change its true data capacity on the fly and dynamically share it between multiple volumes, it must create “pretend” volumes in even-sized chunks. 2TB, 4TB, 8TB, and 16TB are the options currently available. This means that if you have 1.2 TB of actual space, the Drobo will tell your OS you have two or more TB.

This isn’t usually a problem except when you are running low on space (the Drobo is good to warn you when this happens) or when you are using the Drobo as a Time Machine storage device. Time Machine will continue filling up a drive until it is almost full.

In this post, I will provide you with a simple approach that will allow you to isolate your Time Machine data and give it room to grow in the future.

Preparation
If you have an empty Drobo, I suggest you format it with the largest size you feel comfortable with. Drobospace has a good article explaining the tradeoffs of formatting with a larger partition size.

Open Apple’s Disk Utility and click the drive icon (not its nested partition(s)) for your Drobo.
Next, click the Partition tab and examine your data volumes. You should see one or more named segments in the Volume Scheme section.

Most likely you have just one large volume here named Drobo. If you already have more than one volume, we will work with the largest one.

Making the Time Machine volume
If you already have a dedicated partition for Time Machine, you can skip to the next segment.
You should see a small resize handle in the lower right corner of the DLP. If not, you have journaling disabled. See the note on enabling journaling below and then return here.

Press the plus button below the Volume Scheme display to create a new volume. Name this volume something descriptive like Drobo Time Machine. Make sure to use the format “Mac OS Extended (Journaled)”.

Next, drag the volume size divider so your new Time Machine partition will have a much space as you can see yourself giving it down the road. I’ll create an 8TB partition and allocate a 4TB volume to Time Machine.

Finally, press the Apply button and let OSX create the new volume. Note that the Drobo may throw some free space warnings during this procedure. This is merely an effect of the OSX partitioning process. This procedure can take a long time depending on how full and how fragmented your Drobo is.

Give Time Machine only what it needs
Now that you have a volume just for Time Machine, you need to shrink it down to just the size you want TM to use for now.

If I have 1.6TB of usable space available on my Drobo, Time Machine will eventually gobble all that up if I leave the new 4TB TM volume as-is. The final step is to shrink that volume down to the right size for now and to expand it only when you are ready.

In Disk Utility, your new volume should have a small resize handle in the lower right corner. Grab that handle and move it up to shrink the volume. I want my Drobo to always have at least 1TB available for usable storage, so would shrink the TM volume down to 600GB.

Press apply and let the Mac resize the partition. Your Time Machine volume is now the perfect size for your backup needs. When you need to enlarge that volume in the future, just go back into Disk Utility and drag the volume handle down to the desired size.

A word on journaling
If the options to resize or split a partition are disabled in Disk Utility, your drive is either not formatted as Mac OS Extended or has journaling disabled. Having the wrong format type will require a volume reformat in order to continue. No journaling is a quick and simple fix:

Open Terminal.app
Run the command “diskutil enableJournal ‘/Volumes/My Drobo’”
Substitute “My Drobo” for the appropriate name.

Once you have completed the volume creation and resizing procedure, you probably won’t need to re-disable journaling. If you have a good reason to turn it back off, run the command “diskutil disableJournal ‘/Volumes/My Drobo’”.

There was a time several months ago when Apple Time Capsule devices required connected Drobos to have journaling disabled, but that problem has long since been fixed. Journaling should be enabled on all Mac OS Extended volumes unless you know what you are doing.

For more information, Apple provides an excellent KB article on the topic.

Summary
I hope this explainer proves useful to you if you find yourself in this situation. Please feel free to share your experience in the comments below.