I've just received new Dell R730xd 2U server with faceplate 12 * 3.5" drive bays + 4 * 3.5" mid-body tray, located above RAM modules & CPUs. I've plugged 16 * HGST Helium 8Tb 7200 RPM SAS 12Gb/s drives and started 2 * 8 * 8Tb RAID6 volumes background init.
I query drives temperature with smartctl. While front drives are expectedly cool in range of 33C to 37C, the mid-body drives #14..17 are 45C, 46C, 51C, 54C - the latter I'm the most concerned about it is overheating. Init was going on for just a few hours.
iDRAC reports inlet air is 22C and outlet is 44C. Fans rotate at ~4.3k RPM. They spin at ~15k if the lid is off.
Thermal imaging shows #17 is the hottest with the case temperature at 47C.
I'm not yet sure if there's anything with the particular drive or is it about the drive location - will verify by deleting VD and swapping two drives places - will update this post with observations.
Mfg specs say normal operational ambient T is up to 60C (link)
In my view increased temperature affects drive longevity.
However two flex bay rear drives on my older R720xd are 15kRPM and were always around 55C, still alive after 3+ years.
In addition I've requested HGST support for their position.
Another topic on serverfault points to Google research, stating T is a factor after a few years. (link)
UPD1 (20151102): Manufacturer replied quickly: "This drive can operate in temperatures between 5 - 60 C. The drive should normally operate below 50C. If it is operating at a stable temp of 55C then it is running a bit hotter than normal, but still in a safe range."
UPD2: I swapped #14 and #17 places - overheat is location-specific, right-side (looking front to back) is warmer than the left side and former #14 at #17 seat was showing top 56C and former #17 in #14 seat was cool at 40-45C. Adjusting iDRAC->Hardware->Fans->Setup->Fan Speed Offset to "Low Fan Speed Offset (+23%)" (6.8kRPM idling vs 4.4kRPM default, doing RAID init) brought top temperatures for #14 and #17 from 49C and 54C to 40C and 47C. Setting fans to 15kRPM (by setting default reaction to 3rd party PCI cards - I have one) brings temps to 34C and 39C at cost of extra +120W power usage (340W vs 230W).
Of course I'm not using Dell-approved disks. There are no 8Tb drives offered by Dell for this server now, and 6Tb SAS are $830 a piece. I've got 8Tb Helium SAS for $498 bringing pre-RAID TB cost from $138 to $62. Later I realized that Dell-firmwared (and supported by Lifecycle Controller) may be in better communication with cooling and are also getting firmware updates via LC.
Another pleasant surprise for me - swapping #14 and #17 didn't result in RAID rebuild - controller just picked up disks at new locations without saying a word in logs.
UPD 20160426: Now having deployed multiple of R730xd with 12+4 equipped with HGST 8T 12G SAS or Seagate 8Tb 12G SAS, I observe in all of them #14 is ~10C cooler than #17 and partial remedy to bring it to 40-47C range is to increase fans speed setting in iDRAC to +30%.