The data space and other service such as iSCSI target and target USB(*1) are allocated at RAID volume.
In default, the data space is 95% when you create the RAID volume, therefore, the left space that you can create iSCSI target is up to 5%. For example, when you create a 2TB*8 RAID-5 Volume, the default data space is 95%, that means only 14TB*5% = 0.7TB is available for iSCSI.
The data space could be expand, but can not reduced. If you need larger space for iSCSI target, you may need to remove the RAID and then create a new RAID with smaller data space.
*1, Only N5200PRO and N5500 provide target USB function.*2, The maximum LUN size supported by N5200PRO is 2TB.
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