Back in late 2011, I wrote about the use of low-cost NAS devices as mission critical file shares. That blog post generated quite a bit of discussion with our Pro-Partner technicians around the country. While most agreed with the premise of the article, several others asked a related question:

If my client is running low on server disk storage, can I just purchase an external USB disk and attach it directly to the server?

While this seems like a reasonable solution, and it may work for some applications, its probably not the way you want to go with a mission critical server. While external storage (and NAS) devices are good for data backup and archival, they may not be great for a busy server environment.

All disks are not created equal.

A small  (2.5 inch) SATA (Serial ATA) mini-disk which gets power from an external USB port, is typically “spinning” at 5,400 RPM (rotations per minute). These disks can handle a workload of approximately 65 IOPS (input/output operations per second).

Go with a larger (3.5 inch) SATA drive with an external power supply, and you’ll get 7,200 RPM which equates to about 75 IOPS – a 15% improvement.

While these may sound like reasonable numbers, keep in mind that many business-grade servers have “beefier” throughput rates that rely on SAS (Serial Attached SCSI) disks. These disks are typically spinning at 10,000 or 15,000 RPM and have individual IOPS of approximately 140 and 175 respectively.

In other words, an external disk connected to a USB port could have a throughput rate which is less than 50% of the original internal disks. (Ouch!)

If you run low on server disk storage, and cleanup is not possible, your best bet is to add additional internal disk (if possible) or to migrate your existing data to new internal disks of a larger capacity – with the possible aid of a high-capacity NAS device !

While a faster internal disk may cost a little more than an external SATA drive – users will likely feel the difference if you go “cheap”.