Finding information on working RAID solutions for the new DirecTV HD Tivo is difficult, so I thought I'd post a summary of what I've found, my experiences, and list my working configuration. (Scroll down towards the bottom if you just want a final configuration.)
First, some background:
1) The DirecTV DVRs (HR-2x) are finicky about what eSATA enclosures/drives they will work with. There are several threads discussing this, but for a single drive (non-RAID) the most prevalent working boxes appear to be the Antec MX-1 and the Thermaltake Max 5. I've used both and can confirm that they do indeed work and are stable.
2) RAID using consumer (non-enterprise/server) rated drives can be problematic, and is related to drive error recovery algorithms. This seems to be especially true for RAID-5 systems. I've had experience with one RAID-5 enclosure (the Buffalo Drivestation Quattro) and could NOT get it to function with an HR-21. I could not determine if this was an eSATA compatibility issue or a RAID-5 problem.
However, RAID-0 and RAID-1 do seem to work well with consumer drives, and at least one company (Western Digital) explicitly states that their drives are compatible with this configuration.
3) Max capacity for HR-2x series units (including the THR-22) is 2GB.
Now my experience:
1) I'd decided to use a RAID-1 configuration with 2 x 2GB drives. This gives data redundancy (which was my reason for going with RAID) and the maximum possible capacity.
2) Based on hints found in various forums, I selected the Sans Digital MS2UTN+B eSATA RAID enclosure.
3) Also based on various discussions I initially selected the Western Digital Caviar Green 2GB (WD20EARX) drives.
4) Installation of the drives into the MS2UTN was easy. I selected RAID-1 (amongst several options) using the rotary selector on the back and powered it up. I used the USB port to connect it to a PC, and using the Sans Digital supplied software I confirmed the RAID settings and status of the unit.
5) I then powered down the THR-22 and connected the MS2UTN to the DVR eSATA port. I rebooted the THR-22 and it recognized the new "drive", and downloaded/installed the Tivo software. (NOTE: This takes a while and there may be no indication (lights, etc) for several minutes that it has even turned on -- be patient.)
6) I went through the Tivo setup process, created some season passes, and recorded some shows. I thought I was home free, but a few hours later the recordings began to glitch and then the THR-22 began rebooting frequently. (Being a longtime user of Tivos and having nursed a HR10-250 along for several years I recognized this as probably being symptomatic of a drive problem).
7) I reconnected the MS2UTN to my PC and using the RAID managment software found that the array was reporting that it need to be rebuilt. I attempted to do so, but after several hours the rebuild failed. Did some more research on the WD drives and found references to an "Intellipark" function that caused problems with RAID systems. I then downloaded WD software and and turned off this function, but the problems remained -- array would not sucessfully rebuild. I download WD diagnostic software and ran extensive drive tests and found that BOTH drives had failed.
8) I didn't want to guess if the problems were RAID incompatiblity as well as bad drives, so I returned the WD drives to try something different.
9) Next up: Seagate Barracuda 2 TB 5900RPM, ST320005N4A1AS-RK. (I selected the retail kit version since this gives a 5-year warranty. The same drive is available as ST2000DL003 with a 3-year warranty.) This time, having learned a lesson, I ran extensive tests on the drives using Seagate's diagnostic tools BEFORE putting them in the RAID box. As before, I verified the RAID configuration using a PC, then connected it to the THR-22 eSATA port and power-cycled the DVR. DVR found the new "drive" and initialized it, then I went through Tivo setup, etc.
10) This configuration has worked flawlessly for over 2 weeks.
MY WORKING CONFIGURATION:
1) RAID enclosure: Sans Digital MS2UTN+B (in RAID-1 mode)
2) Hard drives: 2 x Seagate Barracuda Green 2TB (ST320005N4A1AS-RK)
1) The Sans Digital MS2UTN fan is loud. Even though their advertising and manuals talk about temperature-sensitive cooling, evidently a recent firmware change forces the the fan to high. My solution was to buy a Zalman FANMATE-2 which allows me to control the fan speed. (Just connecting the 12V fan power to 5V, a usually successful strategy, did not allow the fan to start.) Note that the Zalman connectors are standard sized for a motherboard 3-pin fan port, and will not fit the micro fan controller header in the MS2UTN. I solved this by cutting off the fan connector and the FANMATE connectors and splicing the appropriate wires to make it compatible. The FANMATE sits on the back side of the enclosure using (provided) double-sided tape. At the low end of the range the MS2UTN fan is inaudible but moves enough air to keep the drives cool. (It probably helps that these are "green" 5900 RPM drives).
2) Since the MS2UTN uses the Silicon Image 5744 chipset I would think that any RAID box based on the same chipset would also work for this configuration, especially if it lists the same RAID modes provided by the MS2UTN (RAID-0, RAID-1, SAFE33, SAFE50, JBOD). I think Cavalry and Lacie had enclosures fitting this description, but I don't know their availability and haven't tested them.