The Nexstar MX is virtually a fire-and-forget enclosure. There are some switch jumpers inside the unit which switch it between individual, JBOD, RAID0, and RAID1 storage modes. This interface is very unintuitive for the non-technical user, and will certainly scare off some potential users. However, the manual contains detailed descriptions and diagrams showing the user how to position the switches for the desired mode.
All of these storage modes make the device appear as a single drive to the operating system, with the exception of individual mode. JBOD places data on disk one until it is full, then immediately moves to disk two. RAID0 essentially alternates data placement on the two disks, allowing both disks to spin simultaneously. This increases the effective read and write speed of the combined drive. RAID1 cuts the amount of storage space in half when it writes each bit of data to both hard drives. This mode prevents data loss when one of the hard drives dies.
For testing, I used HD Tune Pro on Windows XP 64-bit. I tested on it using USB2.0 only, as my installation of Windows apparently doesn't like eSATA devices.
Read 23.5 MBps average, 15.5 ms access time, 16.5% CPU usage
Write 24.5 MBps average, 14.9 ms access time, 15.7% CPU usage
Read 26.1 MBps average, 14.8 ms access time, 12.6% CPU usage
Write 25.2 MBps average, 14.7 ms access time, 8.5% CPU usage
Read 16.8 MBps average, 26.8 mb access time, 6.0% CPU usage
Write 10.8 MBps average, 28.8 ms access time, 25.8% CPU usage
I tried both the USB and the eSATA connection on Ubuntu Linux 8.04.1 64-bit. hdparm -t /dev/sdc showed approximately 27 MBps transfer for both when in both JBOD and RAID0 modes, and approximately 16 MBps for RAID1 mode.
It's unfortunate that the RAID1 performance on this device is abysmal compared to the JBOD and RAID0 configuration. I believe the Nexstar MX uses the same chipset as the recently-reviewed Sans Digital MobileSTOR (same device in HD Tune Pro). That unit kept up the pace.
However, the JBOD and RAID0 speeds are sufficient.