I have successfully replaced a failed hard drive in one of my RDR-HX900's, thanks to the information on another thread in this forum. (Search for the model name and you should find the thread.)
One day the drive started making loud clicking noises, and "HDD ERROR" was flashing on the front panel. I tried rebooting and reformatting as instructed in the user manual, but that didn't help.
I tried calling Sony Support. Got a clueless guy in India that was obviously reading from a script. He was of no help, except to confirm what it said on the Sony website, which is that I had to ship it to Texas for service. No place locally would handle it.
Since it was out of warranty anyway, I decided to open it up and see how difficult it would be to replace the drive myself. It was right on top and came out easily with a screwdriver (although the data cable was taped to the top of it, so I carefully untaped it).
I bought a replacement drive at CompUSA as close as possible -- same manufacturer (Western Digital), same specs (ATA 100, 7200 RPM). Put it in and reformatted by holding down the HDD button on the front panel. Reformatted okay, but upon reboot it was still flashing "HDD ERROR".
I then found on this forum how to enter "Service Mode". Did that, and used the "Format" option from Service Mode, and then told it to write the ID to the disk. This solved it, the new drive now works.
[A side note. Once I got it working, I found that all the old recordings scheduled, all options, and even some of the TV Guide programming was still there. So obviously it must have some other non-volatile memory in there, since apparently it doesn't use the hard drive to store that information.]
Since there was only a small cost differential, I bought a bigger disk (320GB) than the original (160GB), hoping it would just automatically use the extra space. However, as others have noted, it doesn't recognize the bigger size and still uses only 160GB. In my research I found that Sony, before apparently abandoning the DVR market, released models HX915, HX1000, and HX1010 outside of the U.S. with bigger drives (up to 400GB), so I'm guessing there's probably some way to tell it to use the larger drive.
If anyone figures out how to get it to recognize the extra disk space, I'd love to hear it. However, I'm happy just to have this unit working again without having to ship it to Texas.
Also, by the way, does anyone know why Sony appears to have abandoned this market? These models all seem to be discontinued, and I couldn't find any hard drive recorders listed on their website any more. Is the competition from Tivo and the cable and satellite companies too much or something? Maybe they got tired of all the complaints about the problems with the TV Guide system resetting itself? Just wondering, because I do actually like the product.