Drobo wants to replace a drive--but NOT the smallest one

Hello all.

Drobo is low on capacity and is saying to replace a drive–fine.
However while I’d expect it to ditch the smallest one first, that’s not the case.
Should I just replace the smallest one anyway?

PHYSICAL DRIVES
6TB
6TB
4TB - it makes (humanly intuitive) sense to replace this one, but
8TB - Drobo wants to replace this one
8TB

MORE STATS
OS: Windows 10 Pro
Device: Drobo 5D
connection: USB 3.0

Drobo Dashboard reports
21.72TB capacity
20.60TB used
1.11TB free

Volumes: 2 (16TB each), drives O: and N:
Naturally with Drobo reporting drive capacity at 32TB, Windows thinks there’s plenty of room; with 13.91 and 6.68 TB used of Drives O and N, respectively.
(this odd quirk of Drobo volume sizing is not my favorite–and I suspect I’m not alone in that regard)

OBSERVATIONS AND QUESTIONS
Can I just replace that 4TB with an eight and be good?
If so, why doesn’t the Drobo think so?
I wish Dashboard could illustrate volume distribution across physical drives–that might preclude this question post.

Obviously I’ll get a 10TB (or 12) drive if I can only retain my data by obeying the Dashboard directive.

However if someone can make sense of a situation that doesn’t make sense on the surface, I’d appreciate that on top of the “What Should I Do Now” recommendation.

Thanks for reading.

The answer lies in your comment that you have 2 volumes of 16TB capacity. Likely that one of these volumes is reaching full capacity and data has been distributed across the 8TB HDD, hence the warning there.

If there is a reason to maintain your 2x 16TB volumes then follow the Drobo recommendation.

You have another option - assuming you have an external copy of all data on your 5D, you can reset the entire disk array to a single 64TB volume capacity and then repopulate it with all your data, which would use the HDDs space more efficiently.

Be aware that increases in HDDs usually need to be done in pairs (not always), so if you upgrade 1 HDD, the Drobo may then ask to upgrade another soon thereafter for replication space, so be prepared.

Awesome, thank you TwinTiger.

HDD upgrades usually happen in pairs

Not entirely unsurprising, but very worth knowing–thanks for the tip.

64T volume

This is not my Drobo (I’m the I.T. guy). Last time I was aware, 16T was the max size for a virtual volume–very glad to know that has increased. I’ll suggest an offload / delete project so we can rebuild if possible.

Depending on that answer, I’ll be ordering three 8TB drives… or a couple fourteens.

Thanks again!

To access the 64TB virtual volume setting, make sure both the 5D’s firmware and the Drobo Dashboard are up to date. Cheers