JBOD (for "just a bunch of disks," or sometimes "just a bunch of drives") is an array of hard disks that haven't been configured according to the RAID (redundant array of independent disks) system.
The RAID system stores the same data redundantly on multiple disks that nevertheless appear to the operating system as a single disk.
Although JBOD also makes the disks appear to be a single one, it accomplishes that by combining the drives into one larger logical one.
I still don't get why you think you need over 400 GB drive for the OS
my system was on a 60 GB SSD to start with but after a few months it was about 2/3 full so I cloned it to a 120 GB drive and its less than half full...all the drives are SSD
thats 2 120GB drives and a 60 GB drive
all I can say is once you get the OS on a SSD you would never go back to mechanical drives again except as a storage medium...they are just F-A-S-T