JBOD (Just a Bunch of Disks) is not a true RAID level, but it’s included as one of the RAID types supported by OS X and the Mac. JBOD is a term that covers many of the non-standard RAID types that many RAID controllers are capable of supporting. Apple’s Disk Utility can use one of the popular JBOD types, concatenation, to combine multiple hard drives into one larger virtual disk.
Concatenation, also called spanning, allows two or more hard drives to appear to a Mac under OS X as a single larger hard drive. This capability can be very useful when you have multiple small hard drives but need a larger storage area for a specific application.
When two or more drives are concatenated, the formatted disk space of each drive that is a member of the concatenated array will be combined. For example, a JBOD array containing two 80 GB hard drives that have been concatenated will appear to your Mac as a single 160 GB drive. A concatenated JBOD array consisting of an 80 GB drive, a 120 GB drive, and a 320 GB drive would appear as a single 520 GB hard drive. Drives in the JBOD array do not need to be identical, or even made by the same manufacturer.
JBOD offers no speed increase, such as RAID 0 provides, nor any increase in reliability, as offered by RAID 1. Should a JBOD array suffer the failure of a member of the concatenated set, it’s possible to recover the data remaining on the other members, although it will probably require the use of data recovery utilities.
Even though there is a likelihood of data recovery, you should plan on having a good backup strategy in place before using a JBOD concatenated set.
To meet my need for a 500 GB hard drive, I used JBOD concatenation to combine two 250 GB hard drives into one large virtual disk.