I found nothing on the internet about the read write speeds of drives (hard drives, SSD) using optical drive bay caddies, therefore I decide to do some tests myself.
I have a Vostro 3450 and recently bought an off-lease Latitude E6410. They both run Windows 7 The optical drive bay of both laptops are removable (more conveniently on the Vostro). I bought the optical drive caddies on eBay:
Both caddies can take drives (HDD, SSD) up to 9.5 mm thick.
First of all, I want to find out what interface the optical drive use to communicate with the laptop. I use Speecy ( http://www.piriform.com/speccy ), a system hardward information freeware to find this informaton. It turns out that this is SATA II (transfer speed 3 Gbps = 384 MBps) on the Latitude E6410 and SATA III (6Gbps = 768 MBps) on the Vostro.
The optical bay caddies of both laptops are "hot swappable" (ie. can be inserted / removed on-the-fly without rebooting, and the new drive will be automatically detected / deleted by the operating system). This is easily done on the Latitude which has an Eject button on the DVD drive (and the caddy I bought - there are models without the ejector). You need to remove one screw on the bottom panel of the Vostro, remove another screw that holds the drive in place, and use a small flat-head screwdriver to push at one corner of the original drive so that it disengages from the connector. This is a little difficult and discourages frequent swapping of the caddy with the original DVD drive.
I tested two hard drives and one SSD:
5400 RPM - Seagate Momentus 5400.6 500 GB (ST9500325AS) - SATA II, max. "internal transfer speed" 147 MBps; max. "external transfer speed" 37.5 MBps (from manufacturer spec sheet)
7200 RPM - Seagate Momentus Thin 500 GB (ST500LM021) - SATA III, max. "sustained transfer speed" 135 MBps; max. "IO transfer speed" 600 MBps (from manufacturer spec sheet)
SSD - Kingston SSDNow V300 120 GB - SATA III, max. read / write speeds 450 MBps
I use ATTO Disk Bench ( http://www.attotech.com/disk-benchmark/ ) to test the read and write speeds. It is not as "professional" as eg. IO Meter, but it has nice display which can easily be understood.
This is the result:
(1) 5400 RPM HDD (read / write speed in MBps):
Latitude E6410 - 74 / 71
Vostro 3450 - 74 / 74
(2) 7200 RPM HDD (read / write speed in MBps):
Latitude E6410 - 139 / 139
Vostro 3450 - 101 / 100
(3) SSD (read / write speed in MBps):
Latitude E6410 - 257 / 239
Vostro 3450 - 492 / 379
(I don't know why the 7200 RPM drive is faster in the Latitude than the Vostro (a newer model). The Latitude has Intel Rapid Storage Technology and the Vostro does not. I dunno if this makes a difference)
You can see the screen shots of the ATTO Disk Bench result here:
Both caddies work as advertised. The SSD is faster in the caddy of the Vostro than the Latitude, because of the difference in speed of SATA III vs SATA II. As expected, the speeds are nowhere near the maximum speed of the SATA III interface or the speeds quoted by the SSD manufacturer. Still it is really fast!
Many people replace the original HDD with a faster SSD, and buy a caddy to use the HDD as a second storage drive. It does not really matter what interface the caddy has (SATA I, II or III), because the read / write speeds of the fast 7200 RPM HDDs are nowhere near the speed of SATA I (1.5 Gbps = 192 MBps). The transfer speed in the caddy (and eSATA, USB 3.0) will be much faster than an external USB 2.0 drive enclosure.