A JBOD RAID set or array, also known as a concatenated or spanning RAID, is one of the many RAID levels supported by OS X and Disk Utility. JBOD (Just a Bunch Of Disks) is not actually a recognized RAID level, but Apple and most other vendors who create RAID-related products have chosen to include JBOD support with their RAID tools.
JBOD allows you to create a large virtual disk drive by concatenating two or more smaller drives together. The individual hard drives that make up a JBOD RAID can be of different sizes and manufacturers. The total size of the JBOD RAID is the combined total of all the individual drives in the set.
There are many uses for JBOD RAID, but it’s most often used to expand the effective size of a hard drive, just the thing if you find yourself with a file or folder that is getting too large for the current drive. You can also use JBOD to combine smaller drives to serve as a slice for a RAID 1 (Mirror) set.
No matter what you call it - JBOD, concatenated or spanning - this RAID type is all about creating larger virtual disks.