You may also want to look into RAID for your design. There are various forms of RAID, but you would do well to look into RAID 5, which would allow you to be writing files to different drives at the same time, but the data would still be on one volume... This gives you several benefits:
1) You are creating a backup as you go. This allows you to have a drive crash, and you can recover. RAID 1 would create a mirrored copy, but 5 involves striping - RAID 1 would only give you the benefit of that backup... though 5 would be more involved, and you would need more drives to set it up (minimum of 3, versus the 2 needed for RAID 1), you have other benefits.
2) The striping also increases performance, Because you can be writing multiple files at once (estimated 3 per second, above...) the striping would allow the files to be "distributed" along the disks, and each disk only taking a part of the burden. The more disks involved, the lighter the burden per disk, but there would be a point where you would reach a limit of performance versus cost...
3) If you back up the data, the backup can take place without degrading any write performance - depending on the size of the cache of the disks, of course, and the form of backup... but for the most part, you wouldn't need to shut down to invoke the backups.
Also, the way you have the system set up, it even sounds like backups would be easier for you - you only need to back up the 24 hours' data at a time, as the filse aren't being modified later. You could even write a batch job that compresses the data if you were concerned about the space taken up by the files... XML is mainly text, so the compression ratios would be high, and decompression would be rarely needed, on only 3% of the files... so you could include compression on the drive without any fear of decompression time. This would also reduce the block sizes needed, and could further increase the efficiency of the system, with the CPU involved in compressing the data, and not just being the go-between of data. (I.E. If all you did was store data, it'd be a waste of that nice CPU processor in that system... but if it were using the "wasted" clock cycles, compressing the data and more efficiently distributing to the drives, all the better!)
With compression, your 2K blocks would probably hold your 8.5K files without a problem. Add striping and RAID backup, along with a hefty CPU, enough memory to not cache any running programs (if any cache is used at all), and you're well on your way to a good system for what you are looking to do.