External USB drive keeps disconnecting randomly I use a Mac mini as a Server and have two OWC MiniStacks attached to it. The Ministacks are connected via USB3 and are underneath the Mac mini. The Ministack in the middle started disconnecting randomly and I need to turn it off and back on to get it connected.

The Mac mini is always on, uses El Capitan and I use the USB drives as Server provided networked disks. The drive disconnects after a day or sometimes a week. I only have it mapped to Server.

I have Seagate Enterprise Capacity Drives everywhere.

I get the following output in Console:

22/08/16 19:08:19,000 kernel[0] USB device 1E91B0B114700000 - BOT reset completed with kIOReturnTimeout.
22/08/16 19:08:19,000 kernel[0] USB device 1E91B0B114700000 - will be reset!
22/08/16 19:08:20,000 kernel[0] 1533181.822274 PRT7@14700000: AppleUSB30XHCIPort::resetAndCreateDevice: failed to create device after (1) tries, disabling port
22/08/16 19:08:21,000 kernel[0] USB device 1E91B0B114700000 - fConsecutiveResetCount = 1.
22/08/16 19:08:21,000 kernel[0] disk3s2: device/channel is not attached.
22/08/16 19:08:21,000 kernel[0] disk3s2: media is not present.
22/08/16 19:08:21,000 kernel[0] disk3s2: media is not present.
22/08/16 19:08:21,000 kernel[0] disk3s2: media is not present.
...
22/08/16 19:08:21,000 kernel[0] disk3s2: media is not present.
22/08/16 19:08:21,000 kernel[0] hfs: err 6 reading VH blk (vol=SERVER DATA 2)
22/08/16 19:08:21,000 kernel[0] disk3s2: media is not present.
22/08/16 19:08:21,000 kernel[0] hfs: err 6 reading VH blk (vol=SERVER DATA 2)
22/08/16 19:08:21,000 kernel[0] disk3s2: media is not present.
...
22/08/16 19:08:22,000 kernel[0] hfs: err 6 reading VH blk (vol=SERVER DATA 2)
22/08/16 19:08:22,000 kernel[0] hfs: unmount initiated on SERVER DATA 2 on device disk3s2
22/08/16 19:08:22,000 kernel[0] disk3s2: media is not present.
22/08/16 19:08:22,000 kernel[0] hfs: err 6 reading VH blk (vol=SERVER DATA 2)
22/08/16 19:08:22,000 kernel[0] disk3s2: media is not present.
22/08/16 19:08:22,000 kernel[0] hfs: err 6 reading VH blk (vol=SERVER DATA 2)
22/08/16 19:08:22,000 kernel[0] disk3s2: media is not present.
22/08/16 19:08:22,000 kernel[0] jnl: disk3s2: do_jnl_io: strategy err 0x6
22/08/16 19:08:22,000 kernel[0] jnl: disk3s2: write_journal_header: error writing the journal header!
22/08/16 19:08:22,775 fseventsd[61] disk logger: failed to open output file /Volumes/SERVER DATA 2/.fseventsd/00000000088fc0e6 (No such file or directory). mount point /Volumes/SERVER DATA 2/.fseventsd
22/08/16 19:08:22,776 fseventsd[61] disk logger: failed to open output file /Volumes/SERVER DATA 2/.fseventsd/00000000088fc0e6 (No such file or directory). mount point /Volumes/SERVER DATA 2/.fseventsd
22/08/16 19:08:23,085 deleted[695] normalizeUserMountpoint:791 volRoot failed for /Volumes/SERVER DATA 2
22/08/16 19:08:23,085 deleted[695] _validateVolume:813 unable to normalize volume: "/Volumes/SERVER DATA 2", vol: (null)
22/08/16 19:08:26,148 certadmin[97828] Connected to the Notify Service


What is going on?
Failing HDD?
Failing Enclosure?
Screwed up Mac mini settings?

Thank you very much in advance!
After studying my problem further, I identified the Newertech Ministack 2012 as the cause of my problem. 

Our drives disconnected, because they overheated. Practically, while fans start spinning when I turn the Ministack on, they do not resume spinning when drives return from standby/idle and stay off. This caused already a couple of dead hdds in our setting and other drives experienced temperatures around 70C.

Strangely the enclosures are quite clean inside, this could mean that the fan behavior has been consistently this way since we bought them. The enclosures have been bought through official channels between 2014 and 2015.

I am starting to believe this might be a production related or architectural related problem this model has. The day after I contacted Newertech the enclosures where unavailable for purchase. Usually I had good experiences with the company, but data storage is very important so I am trying to get to the cause of this issue. 

I tested different settings by turning the enclosure on, accessing the drive, letting the drive idle (usually 10 min) and go into standby, reaccessing the drive and inspecting fan behavior. I switched HDD vendors, cable, interfaces, computers, OS-X versions, this leaves the enclosure.

Could someone please give me a feedback? I would like to figure out if my enclosures just all went bad with the same problem or if we all have a very huge problem! My drives heated up to 70C and should be around 40C, with 60C the maximum advised temperature from the drive vendor.