TRC are seeing an increase in the number of failures of multi disk external storage units. These units employ two or more standard 3.5" desktop hard drives configured into an array of either RAID or JBOD.
RAID Arrays are common in higher end external devices. The more simple external units on the market tend to offer a choice of either RAID 0 or RAID 1 and are normally limited to a maximum of two physical hard drives by design. There are, however, larger units available that support 3 or more physical disks.
In these larger units it is not uncommon for hardware to support multiple RAID levels, although RAID 1, RAID 0, RAID 5 and JBOD are by far the most common forms of array that are supported.
Tolerance against failure on these devices depends very much on the number of disks and the RAID type used. In most cases of external mass storage devices that support multiple disks there is a tolerance of 1 disk failure. It is very unusual to find devices that have a greater tolerance against failure than this but some 4 disks units do allow for the creation of a 4 disk RAID 1 array.
JBOD, is a non-RAID array type that is used for data storage. JBOD is an acronym for Just a Bunch Of Disks, and in reality it is just that. Rather than the methods used within RAID arrays to provide speed benefits and redundancy, JBOD simply allows for a larger storage capacity by making use of that available on each of the member drives.
As there is no redundancy within a JBOD a single disk failure can lead to loss of some or all of the data stored on the array.
A SPAN is slightly different to JBOD in that the drives are presented to the operating system as a single storage unit. Spans take the capacity of the first physical disk within the array as being the first portion of usable space within a single logical volume. Once the operating system needs to write data at a point beyond the extent of the first disk, the next disk or disks are used.