Question 1

Does the eSATA connection actually support the port-multiplier?  Mine only shows one drive.  Or, when using eSATA, does the unit have to be set up as a RAID unit before 4 disks are seen as one larger unit?

Accepted Answer

If you are only seeing one drive in the unit, the part that is giving you a problem is the eSATA interface that is in the computer, not the Rosewill RX305.  I have used mine on Dell desktop and laptop units that have built in eSATA ports and they will only recognize the first drive.  If you use an adapter card in the same desktop that actually supports Port Multiplier operation, you have no problems.
This product absolutely works and I have used it in Windows, Linux and BSD Unix.
SYBA SI-PEX40063 PCI-Express 2.0 x2 Low Profile Ready SATA III

Question 2

Will this support the 8TB seagate SMR drives?

Accepted Answer

I used the device with 2TB and 4TB drives.  The description says that it can use 4, 5 TB drives.  I would not try to use larger drives than what it is rated for.  I have tried such things with other devices in the past and found that hard limitations of firmware have made it impossible.
I can't give you a definitive NO, but I would recommend against trying it when the product description specifically states that it is limited to a 20TB total capacity.
That is probably a hardware limitation.

Question 3

Can this unit be setup so that the drives can be seen as one large drive rather than 4 independent ones?

Accepted Answer

It is possible to set this unit up to be seen as a single large capacity volume.  It can be done in two different ways.  
First, you can connect the unit via USB 3.0 and use the operating system to allocate the four individual disks that it sees as a single large volume.  Windows 7, one of the operating systems I tried this with, will allow you to do this as with mirroring or spanning but it will not set RAID-5.  I tried it on LINUX and was able (once the proper software was installed) to set it in any RAID configuration I chose.  The software on Ubuntu and Mint Linux is called 'mdadm' and it is easily downloaded and installed for free.  I also tried this with Free-BSD and it uses something called ZFS which allows disk redundancy at various levels and works with this unit.  The problem with Free-BSD is that it doesn't properly support USB 3.0, so you can achieve much better performance by using eSATA.
Now that we are talking eSATA, there are several eSATA controllers out there that will manage the drives and set them up in a hardware RAID that makes the operating system only see one disk instead of seeing all the disks independently.  I tried two different controllers from Marvell, one that only works in Windows and supports hardware RAID (worked like a champ) and I tried another one that can work in Linux but doesn't support hardware RAID and it works fantastically with Free-BSD.
I have my drive enclosure setup with four 2 TB drives and mounted to a Free-BSD system to extend the size of my ZFS pool beyond the number of drives that were supported internally in my computer.  I now have 9 drives in what is called a RAID-Z2 configuration shared to my network and it has been completely stable for a couple of months now. 
I just wish that they had this enclosure in a 5 drive version.  This Rosewill product has excellent build quality and I would buy one again as well as recommend it to a friend.  You just need to know what it is for and connect it properly.

Question 4

Will this work with the Xbox One?

Accepted Answer

I actually did NOT purchase this product. I was looking for a NAS and found that this is simply a disk enclosure, meaning you would have up to 4 additional drives and drive letters available. The throughput and capacity appear very good. It's just not what I needed.

Question 5

why a 3tb when formated only leave a 2.1 tb?

Accepted Answer

This is a question to ask the drive manufacturer.  It has nothing to do with the enclosure.  I have 2TB drives (four of them) and each drive only formats out to about 1.8 TB.
The other thing is, did you use multiple drives in an array?
Once I set my four drives into a RAID-5 array, I lost the capacity of a whole drive for the "parity data" and that leaves me with only about 5.4 TB of usable space out of four 2 TB drives.
So, from 8 TB of theoretical storage, I get 5.4 TB of usable storage.

Brand	Rosewill
Model	RX305 4B

Material	Aluminum / Plastic
Internal Interface	SATA III
External Interface	USB 3.0 & eSATA
Specifications	Support all 2.5” & 3.5” SATAI/II/III HDDs up to date. Complies with SATA I & II & III specification up to 6Gbps Complies with eSATA specification up to 6Gbps HDD capacity supports up to 20TB(5TB*4) Adopt UASP to enhance maximum performance of SSD Port-Multiplier(normal) mode- showing 4 HDDs respectively USB 3.0 operates at Super(5Gbps) or High (480Mbps) or Full (12Mbps) speeds 8 CM cooling fan

Rosewill RX305 4B - SATA III-to-USB 3.0 & eSATA 4-Bay 2.5” or 3.5” Hard Drive Enclosure - Full Aluminum Case with Removable Fan, Supports Up to 20 TB Capacity (4 x 5 TB)

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Item#: N82E16817182327