The USB port is a bottleneck on the E02. Copying a 4 GB mpg file from my B01 (same as a P21) to my hackintosh with a 10K RPM HD on a Gbps network I get a sustained 36 MBps.
No picture of SATA but you want a SATA cable with a right angle end oriented the correct way (the notch pointing toward the other end of the cable) so that when attached the cable naturally runs to the back of the P21 where the power and ethernet and 3 USB ports are. I cut a notch out of the edge of the back of the case about 1/8" deep and a little wider than the cable. I plug the SATA cable straight into the HD with no enclosure. As I have both a B01 and a P21 set up this way, I took an old 3"x5" index card metal box and cut one end out of it and then drilled holes in the side for mounting 3 2.5" disks in it using M3 12mm long screws and little rubber washers. To power the disks, I took a molex to 2 SATA power connectors and cut the molex off as well as the yellow 12v and paired black ground wires. To the remaining red 5v and paired ground black wires I attached the red and black of a standard USB cable. So I just plug the USB end into the associated B01 or P21 and the SATA end into the disk. Works great.
I also bought serial JST connectors from https://www.sparkfun.com/products/9240 and drilled holes in the backs of my B01, P21, and E02 plugging the JST end into the serial header inside the pogo's and wires through the drilled holes. Then I attached DB9F connectors to them. I also bought a CP2102 on ebay for $2.64 and attached that to a serial cable with a DB9M on the other end. So now I have serial access to each pogo just by plugging the DB9M into whichever DB9F I want.
You can boot off the SATA drive and part of setting that up is copying various files to it including a staging file which determines the clock speed. So clocking at 850 is just using the right staging file. It's documented on http://archlinuxarm.org/forum/viewtopic.php?f=29&t=2146.
I am running mythbackend on my 3 pogo's as well as transmission, a CalDAV server, and serial control for X10 on my E02. The E02 runs a newer linux which runs USB serial better ie USB serial really does not work on the B01/P21's. Running mysql for myth on the P21 and mythfilldatabase on the B01 reduced mythfilldatabase time to 1.5 hours from 4 hours when both were run on the E02 even though the E02 has twice the memory. Disk access made all the difference.
I record as many as 5 channels at a time with an HDHomeRun Prime and an older HDHomeRun clear QAM 2 tuner. I need the disk performance to be able to record that much and watch one at the same time. And that is right on the edge of performance. I record 3 on the B01, record 1 with mysql also on the P21, and record 1 on the E02 also running the other things I mentioned plus mythweb. I am thinking of adding another P21 to spread the load a bit more taking mythbackend altogether off the one running mysql. First I may try just to record 2 channels on the E02 and see how that works.