Unmanaged switches — These switches have no configuration interface or options. They are plug-and-play. They are typically the least expensive switches, found in home, SOHO, or small businesses. They can be desktop or rack mounted.
Managed switches — These switches have one or more ways, or interfaces, to modify the operation of the switch. Common management methods include: a serial console or Command Line Interface accessed via telnet or Secure Shell; an embedded Simple Network Management Protocol SNMP agent allowing management from a remote console or management station; a web interface for management from a web browser. Examples of configuration changes that one can do from a managed switch include: enable features such as Spanning Tree Protocol; set port speed; create or modify VLANs, etc.
Two sub-classes of managed switches are marketed today:
Smart (or intelligent) switches — These are managed switches with a limited set of management features. Likewise "web-managed" switches are switches which fall in a market niche between unmanaged and managed. For a price much lower than a fully managed switch they provide a web interface (and usually no CLI access) and allow configuration of basic settings, such as VLANs, port-speed and duplex.[10]
Enterprise Managed (or fully managed) switches - These have a full set of management features, including Command Line Interface, SNMP agent, and web interface. They may have additional features to manipulate configurations, such as the ability to display, modify, backup and restore configurations. Compared with smart switches, enterprise switches have more features that can be customized or optimized, and are generally more expensive than "smart" switches. Enterprise switches are typically found in networks with larger number of switches and connections, where centralized management is a significant savings in administrative time and effort. A Stackable switch is a version of enterprise-managed switch.
Source: http://en.wikipedia.org/wiki/Network_switch
I would explain in more personal detail, but the wiki explains it pretty well.
The HP MSA1500CS is a pretty wimpy device. I have one, and I hate it. I'm somewhat surprised it has kept up with your stated workloads. It probably comes as no surprise that I recommend upgrading to the MSA2000. It has a much better storage architecture than the 1500CS, and can scale better.
Without more data I can't recommend going to an EVA4400 (HP's 'entry level enterprise array') versus the MSA2000. The 4400 will take you a lot farther than the MSA2000 will in terms of scale out, but I don't know what kind of growth you expect.
RE: LeftHand vs. MSA2000
So long as you have the ethernet network for it, the LeftHand unit should out-scale the MSA2000 by a long shot. The distributed storage controller it uses makes that kind of thing easy. You'll pay more per storage shelf, but you can scale to silly amounts with it. Once you start hitting the I/O ceilings on an MSA2000 (which will depend on the drive technology you use as well as any active/active configs you can use) you're pretty much done. For the LeftHand products that ceiling is a lot more mushy.
Where the LeftHand approach really saves you is with parity RAID. Doing rebuilds after a failure is the most CPU intensive thing it does, and is where my MSA1500cs falls flat on its ass. On my 1500cs, rebuilding a RAID6 array across 6.5TB of disk took about a week, during which time it was deeply intolerant of large scale I/O writes to anything on the array. Since LeftHand has a controller in each cabinet, restriping a LUN on one shelf will not affect performance of LUNs on other shelves. This is very nice!
All in all, if you have the budget for it the LeftHand devices should serve you a lot longer than the MSA2000.
Best Answer
Well there may be obvious differences, such as interface type, rotational speed, etc.
Beyond that, even when two disks have identical entries for the above spec, the drives that come in a SAN will probably be certified/warranted for 24hr continuous operation, and may also include custom firmware from the SAN manufacturer.
As for the price difference - if you're buying a decent (or even semi-decent SAN) you are buying a lot more than a bunch of hard disks - the drives may only be a relatively small part of the cost.