Problem with the R4 as I understand it is that you can't record to the CF card, only to the hd, which is another stupid design flaw.
From what I've read in old posts, it seems that some people have had no problems with the R4 and others have had a lot, with regards to the hdd slow errors. My instinct tells me that it is more of a HD tolerance issue, or lack of quality control. If you get one with a top notch drive then it's all good. I'm just guessing here.
With Minidisco's low price on this, I wish that I could know that it would be rock solid, I'd order one today. I'm on the fence between the r4, hdp2 and the 671.