Mac and Windows report HUGE free space difference on USB DAS?

Stepping in to help a new client that lost their IT person in a tragic car accident. Sad.

That being said, I'm faced with multiple issues. One that is weird (and Mac related I think - not my forte) :

They have a Drobo device (Direct Attached Storage (DAS) via USB). It has (4) 1TB drives running RAID 1 (I think - not sure how Drobo works) - about 2.5TB of usable space. The Drobo reports that it is almost full.
However, a Mac user on the network says that they have tons of space free (14.92 of 17.57TB??)

We both see the same shared folder - any idea why there is such a HUGE difference in what we see?
I did a quick look at the Drobo and did not see anything like thin provisioning, etc.
Just trying to get my head around their entire network, etc. so I don't have much to go on or to offer. Just hoping a Mac person could shed some light on what they think they see as so much disk space - and what I see - almost out of space!
Sal SoricePresidentAsked:
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bbaoIT ConsultantCommented:
how about used space? a simple calculation of total and used space can tell you the real number of free space. then you can know which app gives the correct result, and then we can discuss why.
Eoin OSullivanConsultantCommented:

The OSX Finder Info image you attached is probably WRONG .. if the drives in the Drobo are 1 TB then it is basically impossible to have 18TB. I've used a number of Drobo devices 7-10 years ago and found that the OSX Info Panel was always misleading .. not really geared to understand that type of RAID device as there is some clever software running on the DROBO to manage the hot-swappable drives etc.

There's a Drobo App (Drobo Dashboard) for OSX which you should use to analyse and manage the device .. and run it ideally on the Mac it is connected to.

Also it looks like the Mac is quite a few years old and running an older version of OSX so make sure you download the version of Dashboard for the Drobo model and OSX version as the newest versions probably won't work for you.

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Drobo does something called deduplication and sums up every "copy" of the file.  Those copies are actually hard links to he exact same file that occupies the exact same space on the disk.  This is to reduce the actual duplicate copies and allow multiple time based backups to exist in a much smaller space.   Unfortunately, those hard links exists in multiple folders and when you check the space on each folder, they do not differentiate the hard links and will report the space used by each link in full as if that was the only file.

On the Mac, you can always go directly to the command line to find the real disk usage with df -h.  You will then see the actual disk, rather than the Public folder container.  Also, if you select the Drobo disk mount, instead of the Public folder and do a Get Info, you should see the actual disk usage.
Sal SoricePresidentAuthor Commented:
Thanks all for the comments. It does indeed seem like the Mac Finder is totally wrong. Just not sure why (or how) it could think there is 17.57TB of space on a device with only (4) 1TB disks! I could not find any evidence of thin provisioning, etc. that could account for the huge discrepancy. I'm gong to have the client run the Drobo Dashboard app and try the Mac commands mentioned to verify.
It's not exactly wrong.  It's summing up the files it finds in each folder, including the deduplicated files.  This is actually the correct behavior.  The finder does not know that each file has a hard link to the same data.  You need to do a Get Info on the actual disk, instead of the mounted folder partition to get the Disk usage, rather than the folder usage.

The Windows system is showing the disk to be the Public folder, so it's showing the disk usage versus the folder usages.
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