4 x Seagate ST31000528AS (1 TB) hard drives.
They're in RAID-5, been in use since November 2009.
I've been very happy with the read speed (often 40-50 MB/s). The write speeds are acceptable (about 10-12 MB/s). For my purposes (backup, and some music serving for my sitting-around-the-house mode), it's great.
I bought two from one location, and two from another (when they were on sale).
I'm neurotic about trying to space out my hard drive purchases just to help from ever being bit by a bad batch or something. I know, OCD.
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I've had no issues until recently with the 1.04 firmware. It appears to just be bugs with the firmware itself, and nothing that affects the unit or the data. I know a lot of people have had some "middle of the road" and bad experience with the DNS-343, but I have to say...mine has been really quite great. (With the understanding, of course, that I probably don't demand as much from my unit as others might.)