RAID 5 usable sizeParity data uses up the capacity of one drive in the array (this can be seen by comparing it with RAID 4: RAID 5 distributes the parity data across the disks, while RAID 4 centralizes it on one disk, but the amount of parity data is the same). If the drives vary in capacity, the smallest of them sets the limit. Therefore, the usable capacity of a RAID 5 array is , where N is the total number of drives in the array and Smin is the capacity of the smallest drive in the array.
The number of hard disks that can belong to a single array is limited only by the capacity of the storage controller in hardware implementations, or by the OS in software RAID. One caveat is that unlike RAID 1, as the number of disks in an array increases, the chance of data loss due to multiple drive failures is increased. This is because there is a reduced ratio of "losable" drives (the number of drives which may fail before data is lost) to total drives.
From your link. If all 3 disks are the same size you lose one to parity, ie 33%.
If you have more disks, yes you can minimise the unusable disk space, but lose 2 drives and you lose all data as the parity is broken up.
With unRAID lose 2 disks and you lose 2 disk's worth of data.
Granted write performance is pretty average with unRAID but read is fine. I'm not here to sell it, basically what I use. Read the website and the forum for more.
Isn't it possible to build the LinuxMCE core in that chassi, set up a webmin NAS in it (or is that necessary when using it as core?), and for now only buy a couple of SATA discs to rip to?
Would that be something? Or is it not as great?
Sorry can't help you with that, above my paygrade.