Hello,
i have a Drobo with 2 drives (1,5TB) and Windows (XP prof.) show me 2 Volumes too. Now i have decided to put a third drive (1,5TB) into my drobo. Drobo Dashboard has detected the third drive, but i can´t access this drive from windows - windows only show me the first two volumes (and in windows the capicity didn’t changed from the first two volumes) - what can i do ?
3 x 1.5 TB will yield at maximum 3 TB of usable storage, so unless you set the volume size on your Drobo to 1 TB instead of the XP maximum of 2 TB, you’re only going to have two 2 TB volumes.
Until your usable storage exceeds 4 TB, you’re within the 2 x 2 TB thin provisioning that was set up.
See What is Thin Provisioning? and What volume sizes may I use and what happens if my disk space exceeds my volume size? for more info.
that is correct, that is how drobo works
you never have access to the “drives”
drobo is a virtualised storage pool
you create volumes - and you have access to these volumes.
drobo intelligently spreads these volumes out across the available drives.
the space reported by windows is the space on these volumes.
it is totally separate from the number and the size of the drives you have installed in your drobo.
if you have more available storage (in terms of drives installed) than you have available as volumes, then drobo will create an additional volume to let you access that storage.
i will try to illustrate with a simple example.
if you install two 1TB drives in drobo - you will have 1TB available storage space (because drobo uses 1tb for “protection” in this case")
now with your 1TB of available space you can create a 2TB volume.
notice that you can format the volume to be a different size to the actual physically available space - you could make it 16TB if you like (although some versions of windows dont support volumes this big).
obviously you will never be able to put more than 1TB of data onto it - as you only have 1TB physically available, but you can still format the volume to be a larger size. this is called thin provisioning and it means that you can add additional capacity at a later date without having to reformat your volume (which is what you have done).
so we have two 1TB drives in drobo, giving us 1TB of physicaly available space, formatted with a 2TB volume.
you cant put more than 1TB of data onto your volume, since that would exceed your available capacity, but windows “sees” a 2TB volume, and reports it as having 2TB of available space (you would need to check drobo dashboard to see your actual used/available space). drobo will stop you from adding more than 1TB of data to this volume by warning you when it gets full, and by slowing down more and more as you get closer and closer to your real 1TB limit.
now supposing we were to add a third 1TB drive to out hypothetical drobo.
you slide the drive in.
you now have 2TB of physically available space. (three 1tb drives - and 1TB used for protection)
however windows still only sees that you have your single 2TB volume.
as far as windows is concerned nothing has changed. this is thin provisioning at work.
your formatted your volume to be larger than your actual real space, and now when you have added capacity, your volume has not had to change, but you can utilise this additional capacity.
you can now add 2TB of data to your 2TB volume (since you really do have 2TB of storage available).
lets add a fourth 1TB drive to our hypothetical drobo.
you now have 3TB of physical available space.
since you only have a single 2TB volume, you cant utilise this additional space, so drobo will automatically create a SECOND 2TB volume.
now you have TWO 2TB volumes which windows will see (even thought you only have 3TB of available space on drobo) - again this is more thin provisioning 
drobo will share this available storage between your two volumes. you can never add more than 3TB in TOTAL across your two 2tb volumes - again drobo will slow down.
this is exactly where you are now.
you had two volumes (i will assume 2tb each since this is the default) so windows “saw” 4tb of space, but with two 1.5tb drives you only had 1.5tb of available space. you added another 1.5tb drive, this gives you 3tb of real available space, but since you have two 2tb volumes, your available capacity is less than your formatted capcity (2x2tb) so nothing had to change.
in short, everything is working perfectly and you can now simply add more data to your two volumes than you could before
sorry its long, but i hope its clear!
First of all, thank you for the long post and your arduousness
And that is the point. In the start i had two drives with each 1.5 TB. And during installation Drobo (or I) have created 2 Volumes (in Windows) with each 1 TB (shown in windows)
Now, with the third drive, i have nearly 3 TB free for space (Dashboard say 2.72 from 4.5 ) - so I have more capacity (0.72TB) as windows show me (2 Volumes with each 1 TB).
Because the capacity of my 2 volumes in windows is lower than drobo dashboard shows, in your opinion drobo create automatically create a third drive with 1TB ? That doesn’t happened - so I cant use more than 2 TB with Windows … hope you can understand my problem.
Sorry for my bad English - I am very thankful for your endurance.
Greetings from German Summer City Berlin
If you created two volumes yourself, it may be that drobo is no longer automatically handling volume creation (i don’t have a regular drobo anymore so i cant remember exactly how it works) but it may be at you need to manually create the third volume inn order to utilise your additional capacity
If, when you initialized the Drobo from Drobo Dashboard, you chose the 1TB size option, instead of 2TB, then you should have three volumes.
Check Disk Management and see if there is a volume that is not mounted with a drive letter. If not, run Drobo Dashboard and go to the Advanced information. If it does not prompt you about creating a new volume, put the Drobo into Standby, disconnect the data cable, wait until the lights go out, then power-cycle it.
I have disconnected the drobo - but nothing happened after new start.
I have some screenshots for you, i think it’s than easier to show my problem:
www.goenz.com/drobo1.jpg
www.goenz.com/drobo2.jpg
www.goenz.com/drobo3.jpg
www.goenz.com/drobo4.jpg
on the first screen you see - drobo shows one drive without a drive letter.
If I choose the option to rename a drive you see on screen 2 - that 2 drives have the same letter (J), if I choose another free letter, drobo say, it isn`t possible to rename the drive.
On screen 3 and 4 you can see my two drives at windows with their capicity. Hope you can help me, to get a third drive, shown under windows
Have you rebooted your computer?
Of Course 
Did you use dashboard to format your drobo originally?
Drobo has created a third volume for you, but your drobo2.jpg shows that both Volume 1 and Volume 2 are assigned to the same drive letter, which of course, doesn’t work.
I’m not sure how it happened, but it looks like it happened.
Change the letter assignments and all three should appear.
Good catch. Missed that!
Is that the screen to rename volumes or to reformat?
good question - i 've bought my drobo in 2008 - so i don’t know, what i did 2 years ago.
Should i remove the third drive from my drobo and after all datas are safe, put it again in the drobo and look, what happens ?[hr]
[quote=“bhiga, post:11, topic:1608”]Change the letter assignments and all three should appear.
[/quote]
That is the problem - if I want to change the letter of one, drobo said, he can’t change it
In drobo2.jpg in window on bottom right hand corner. Volumes umbenennen. Is that the rename your volumes or to format your drobo screen?
(Sorry my german is pretty much limited to ein bier bitte and war ist toiletten)
Or I could just use Google Translate. It’s the rename screen.
Remane that 3rd volume, different drive letter and something besides Drobo.
To rename -( i don’t want to reformat (all my datas :O) )
See if you can change one of them from Disk Management (diskmgmt.msc) - right-click and choose whatever the German equivalent of “Change Drive Letter and Paths…” maybe it will let you change the letter from the Windows side.
Renaming will not reformat.
Yeeeeeeessssssssssssss - the problem is solve
In Windows there is a drivingadministration (http://www.windows-tweaks.info/html/laufwerksbuchstaben.html - sorry, german 
) - there my third drive was listet without a letter, but it was shown and i could give the drive a letter - now everything is fine
Thank you for all your great and fast help.
Best wishes from Berlin
goenz
Glad to hear it is resolved!
Definitely a Windows weirdness. I have had similar things occur with USB flash drives and such too.