Spare hot-plug disk trays?

But how often do you really need to swap disks? It shouldn't be very often at all unless you manage a large number of units. If you replace a disk then the replaced disk should be a failing/dead disk or a low capacity disk no longer intended to be used. RAID is not a backup (see Preventing Catastrophic Data Loss). Important data primarily stored on the ReadyNAS should be backed up regularly (e.g. to a USB disk, another NAS or some place else).