@timmo - You mentioned speed as a priority in your last post and unfortunately this is another strike against an expansion box and it is all down to that little single cable again.
For whatever reason these NASes use SATA backplanes so you get SATA drives and the eSATA link to the expansion box. SATA is a bit rubbish at the best of times as it is only half-duplex - it can only send or receive data rather than transport both at the same time, effectively halving throughput.
The main NAS box will (to some degree) lighten the burden of this potential bottleneck by having a SATA interface with each drive and aggregating the bandwidth and I/O of each of your 6 SATA drives together across a SATA backplane that can handle multiple SATA interfaces - hurrah!
Instinctively you know this all anyway so think about it from the point of the 5-bay expansion box. Five drives all wanting mixed read/writes sharing a
single SATA backplane and a
single SATA connection to the main NAS that only moves data one way at a time.
This cluster 'f' of data now has to be managed by the CPU and RAM of the main unit as it tries to juggle this single ugly stepchild of SATA on its backplane that has 5 drives hanging off it rather than the 1 it would rather have. At best you get a 5th of the performance that you would like but in reality it never gets this good as the inter-drive traffic holding the RAID parity together is fighting for the same limited resource.
Even as a distinct volume the clucking expansion box will still drag down the resources of the main unit and if you did something mad like span a volume over the link then the whole assembly would run at the slowest rate governed by that single one-way-at-a-time-sweet-***** eSATA link.
[pst... sell it, buy a proper NAS]