concatenated disk set
A concatenated disk set—also called “Just a Bunch of Disks” (JBOD) or “spanning”—is a single large disk created from several smaller disks. A concatenated disk set acts as one large disk, with the combined capacity of all the smaller disks.
A concatenated disk set is helpful if you have a file, such as a database, that’s larger than any of your disks. It’s also useful if you need to create a mirrored or striped RAID set with one large disk and two smaller disks.
If all the disks in the set are about the same size, consider using a striped RAID set, which lets you access your data more quickly.
After it’s been created, you can use Disk Utility to add more disks to a concatenated disk set to increase its size.
You cannot use some disk management features, such as FileVault disk encryption, with concatenated disk sets.