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i want to add a hard drive caddy to add another hard drive to my mac, and i want to create a JBOD combining the hard drive in the hard drive caddy with the hard drive included with the mac. is it possible? will i encounter problems?

I wouldn’t recommend it. JBOD’s disk sets don’t have any protections. So if either drive has a problem, both drives data is not accessible and useless! As laptops bounce around you are at a higher risk of loosing your data. JBOD disk sets are useful for data sets that exceed a single drives storage space. Like a large image data base or video’s which make up a movie while it’s being edited. Even still these are more often placed on external RAID drives (RAID-5 or RAID-6). With RAID you need to have the same size & kind of drives and they need to use the same I/O. RAID vs JBOD JBOD doesn’t care about size or kind or even the I/O. But, you’ll pay for it! The storage of data is random! So you’ll see spits and spirts within your data access across the drives. The better direction is just get a much larger drive and if you can swing it get a SSD. As far as your system using the optical drive to host a drive … You’ll want to review this page: OWC Data Doubler to see what your system is able to support for drive. Apple has a problem within some of its systems so even though the system appears to support a faster I/O SATA III (6.0 Gb/s) it can’t really do it. In these systems you need to use a fixed SATA II (3.0 Gb/s) drive (one that only runs at SATA II).

It is possible, this function called “fusion drive”. Problem will be, if one of your disk will fail, you lose all data.