Upgrading HDD in Gen2 - which bay should I upgrade, and how large?


Two (hopefully) quick questions:
1- Can someone confirm if the Gen 2 (USB 2.0 / Firewire 4-Bay) can support drives larger than 4TB?

My current configuration is (in order of top bay to bottom bay): 4TB/4TB/4TB/3TB

My Drobo currently has less then 1TB free, and has highlighted the first drive to be swapped for a larger drive, which seems weird to me, as it means the 3TB would still be in the system.

This brings me to my next question:
2- Should I just swap the 3TB for a larger drive, even though the dashboard and machine is telling me to swap the first 4TB?

Ideally I’d like to swap the 3TB for a 6TB. While I won’t use all this space, it seems pointless to spend the money on a 4TB drive, when in the future I’ll have to buy two new drives to create space.

Thanks for any replies (I’ve cross posted this with the direct attached forum as I’m not sure where this belongs)

Since is a double post… I will post on this thread.

  1. I personally would not recommend larger than 4TB drives on Drobo Gen2.
  • with that, for your case… this will limit you to 4TB drive.
  1. It been reported by some users that Drobo Gen2 in some cases… misinterpreted 4TB drives as 3TB. So this explains why Drobo Gen2 is indicating to swap the Top Most drive which is a 4TB thinking that it is a 3TB drive.

*With point (2) … what the firmware thinks is that all 4 drives in the unit are 3TB drives. Therefore it will prompt you to swap the Top Most when low in space.

**But in terms of functional and available disk space, the firmware has no problem with the 4TB drive.

For drive upgrading, although Drobo Gen2 flagged Top most drive. But I believe is ok to swap out the 3TB to larger drive.

Now there’s the details with your current setup 3x 4TB, 1x 3TB regarding disk space upgrade. (is in a rather dis-advantage position)

a) Assuming the Drobo Gen2 is in Single Disk Redundancy (SDR)

  • with 3x 4TB and 1x 3TB, take away ONE drive (largest) for SDR. You left with 2x 4TB, 1x 3TB = 11TB (RAW capacity)

b) Again… assuming Drobo Gen2 works fine with 6TB drive and we saw it with the 3TB

  • so it will be 3x 4TB, 1x 6TB, again… take away ONE drive (largest) for SDR. That will leave us with 3x 4TB = 12TB (RAW capacity)
  • So you can see you do NOT gain much disk space unless you upgrade another drive to 6TB.
    -> 2x 4TB, 2x 6TB … minus SDR … 2x 4TB, 1x 6TB = 14TB (Raw capacity)

**Important note here is to upgrade ONE drive at a time. Swap ONE drive, let Drobo completes its Rebuilding then upgrade the next drive.

The above is all assuming Drobo Gen2 works well with 6TB drive which I personally do not recommend. Better to stick with 4TB MAX.

With that…limitation… upgrading the current 3TB to a 4TB drive, will makes 4x 4TB. And with SDR, total raw space is 3x 4TB = 12TB. You only again 1TB extra from current set-up.

Thanks Don.

Why do you recommend staying away from drives bigger than 4TB? Personal/Forum experience or is it firmware issue?

Based on this I’m thinking on saving my money and investing in external drives for the time being (this is copying a stack of DVD discs I found, so I have the redundancy of hard original media), and upgrading to a the newer four bay generation when it’s needed.

Hopefully at this time 6TB prices have dropped.

My understanding, that to migrate from a 4-bay Drobo to a new one, is straight forward (remove drives and put them into the new machine).


Many fellow users of Drobo Gen2 also recommended to stay with MAX 4TB drive with Drobo Gen2. Drobo Gen2 firmware has not been updated for awhile as its a old model, and is not tested with 5TB, or 6TB drives.

My understanding, that to migrate from a 4-bay Drobo to a new one, is straight forward (remove drives and put them into the new machine).

**IMPORTANT - ALL migration of Drobo to Drobo… has to be done when the Unit is powered OFF.

  • Note: Inserting drive(s) into a Drobo while is powered ON… will auto-format the inserted drives(s). This will wipe-out any existing Data in that drive(s).