Finder (Mac) incorrectly identifying disc size?


New to Drobo and just learning the ropes.

I have a Drobo containing 3 x 500gb drives, formatted as a 16tb volume. According to Drobo Dashboard, 576.11gb are being used, leaving 323.66gb free…

Finder, however, reports (at the bottom of the window) that the Drobo has “16.97 TB Available”. I know Finder sees the Drobo as one big drive, but 16.97 TB is bigger than my formatted volume!

Any ideas?

Thanks in advance,


I think you are a victim of “marketing” megabytes/gigabytes/terabytes (powers of 10 - 10^6, 10^9, 10^12) versus “computer” megabytes/gigabytes/terabytes (powers of 2 - 2^20, 2^30, 2^40).

Newer Mac OS versions measure 1 TB = 1 trillion bytes whereas most other operating systems (except for TiVo’s web interface) measure 1 TB as 1,099,511,627,776 (1024^4) bytes.

So in newer Mac OS versions that use proper base 10 notation, 1,099,511,627,776 bytes = 1.099TB whereas in computer base 2 notation, 1,099,511,627,776 bytes = 1.0 TB


With OS X Snow Leopard Apple have changed the way they show disk size. See http://support.apple.com/kb/TS2419 - This is to be in line with how hard disk manufacturers advertise capacity for hard disks.

Drobo however presents an actual 16TB disk.
If you open up Disk Utility on your Mac and select your Drobo you can view the capacity. Mine shows:
Total Capacity : 17.59 TB (17,592,186,044,416 Bytes)

However if you work it out you will find that
17,592,186,044,416 Bytes exactly equals 16TB

Technically it is 16TiB (http://en.wikipedia.org/wiki/TiB ) but few people seem to know this measurement…

see, i thought it was that at first, but then surely it should show him a 17.59TB disk (since it counts in decimal versus drobo’s 16tb binary) i couldn’t figure out how it came up with 16.97TB so i kept quiet :slight_smile:

now i’ve just realised its 17.59TB minus his used space = since its showing “available”

Thanks, everyone!


Umm … I am new to this as well … and i have been puzzled too by the way 3 x 500 GB can be 16 TB …

Is there a way to make Finder show the actual space on the Drobo in stead of the theoretic max. space?

Kind regards,


its called thin provisioning, and it means what when you add new disks you dont need to reformat drobo (since the partition is already 16TB)

I dont think there is. someone wrote a utility to get windows to show the real space, but i could never get that to work