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!