More Storeage

That would depend on what your requirements are for the storage.

I would lean away from DROBO, they use a proprietary storage system that you can only have attempts of recovery done by them.

you could use something like Openfiler to provide SAN style external storage using various protocols, and limiting which addresses can access it.  

Similar to drobo would be unRaid  http://lime-technology.com/, except it uses standard linux file system for the storage disks.  It is primarily created for user storage of media files - movies and music.  I'm not sure I would use it in production, except maybe for archival storage though.  but you can add drives as you need them.  it is a build yourself server (or you can buy premade systems) free for 2data and 1 parity drive, inexpensive for expansion up to 20 data drives.

Doesn't sound like it would work for us.  We are definitely using it for production in an all Windows environment.  We would be using it for backup purposes also.

OpenFiler would work for that.  it is just software to run on your own hardware.  if your hardware does raid, it will work with that, or you can do software raid with it.

FreeSan is another one similar to openfiler.  Most will not recommend for production, but we have used freesan as a vmware san in production without any troubles for a few years now.  though since getting a full blown SAN, it is now back to development systems.

For direct attached additional storage, you could get external eSata drive bays with various numbers of slots in them.

correction... it is FreeNAS, not freesan.

We are looking for an economical hardware solution since we are running low on disk space on our servers.

If they will be direct attached storage, then I would suggest e-sata external enclosures as an option.  

Regarding the hardware, you may be interested by the 135 TB 4U diy BackBlaze pod at $7384

Thank you for your input.  $7,384 is more than we wanted to spend.  Callandor pointed me to a great site...Thanks