JBOD isn't a real RAID system. It means Just a Bunch Of Disks. A JBOD is a few disks put together as one big volume.
Function of a JBOD
The files are written on the first HDD until it's full, then on the second one and so on.
If one of the HDDs is down, all the data of this HDD will be lost. There's no security at all. However, a JBOD is still less risky than a RAID 0.
The common JBOD failures are :