I just wanted to describe what worked for me, and some of the glitches I had to deal with..
My provider is Long Island Cablevision and the 8300HD has the SARA software.
I bought the Azio External SATA HD Kit from Newegg. This kit comes with the proper eSATA cable, no adaptor needed. It is a well designed little box. I was a little concerned because it has no fan, but so far it just gets warm, not hot.
I got a deal at Costco on a WD 7200 rpm SATA 400gig hard drive. Total cost of drive and enclosure was less than $200. As I said, I did not need to buy any extra cable.
At first it didn't work. I had followed Scientific Atlanta's instructions (get the "Connecting the 8300HDC Digital Recorder" guide from their web site) which say to turn off and unplug the 8300HD before plugging in the external drive, and then fire up the drive before plugging the 8300 in.
The 8300HD detected the drive, but said it was not working properly, check the cables, etc. I unplugged and replugged the eSATA cable and retried it several times with the same result.
Then I reversed the cable and this time it detected the drive normally and asked if I wanted to format it, which it then did.
My unit was at about 87% full when I added the external drive, and that did not change. I set it to record some more programs and waited for it to start using the new drive and the show increased space available, but it didn't happen. The space kept decreasing until it was at 98% full.
At this point I unplugged the DVR and rebooted it. It now gave a message that the external storage device was functioning. I checked the available space and it was showing 27%!
My conclusion is that you may need to play around with this stuff to get it to work, but that it is worth it to put your own drive and enclosure together compared to buying a dedicated drive like the Maxtor. The maxtor is over $300 bucks and I think you only get around 160 gigs, while I spent about $200, and got 400 extra gigs. And I can always use the drive with my computer if I get rid of the 8300HD, whereas the dedicated Maxtor drive will not work with a PC.