USB drives connected to hub disppearing I've got four external USB drives connected to a 7 port powered dlink hub.  All of them are externally powered, save for one of them.  The hub is connected to a late 2012 Mac Mini.

If I connect the drives directly to the mini, they work perfectly.  When I connect them to the hub, they randomly start to disappear and the finder shows me the "You have unsafely disconnected this drive" dialog for each of them.

I am running OS X Mavericks, but this occurred under Mountain Lion as well.

When they disappear I am unable to get them to re-mount save rebooting the machine. Launching Disk Utility actually just hangs until I force quit it, and if I try to mount them with diskutil mountDisk /dev/disk9 or similar I get One or more volume(s) failed to mount.

They are sort-of visible in the diskutil list command though.  I mean sort of since not all of them are listed there and some of them are listed twice (e.g. the drive named Hobbes is on there as /dev/disk7 as well as /dev/disk3.

Here is the output from diskutil list.

/dev/disk0
   #:                       TYPE NAME                    SIZE       IDENTIFIER
   0:      GUID_partition_scheme                        *500.1 GB   disk0
   1:                        EFI EFI                     209.7 MB   disk0s1
   2:          Apple_CoreStorage                         499.2 GB   disk0s2
   3:                 Apple_Boot Recovery HD             650.0 MB   disk0s3
/dev/disk1
   #:                       TYPE NAME                    SIZE       IDENTIFIER
   0:                  Apple_HFS Calvin                 *498.9 GB   disk1
/dev/disk3
   #:                       TYPE NAME                    SIZE       IDENTIFIER
   0:      GUID_partition_scheme                        *1.0 TB     disk3
   1:                        EFI EFI                     209.7 MB   disk3s1
   2:                  Apple_HFS Hobbes                  999.9 GB   disk3s2
/dev/disk4
   #:                       TYPE NAME                    SIZE       IDENTIFIER
   0:      GUID_partition_scheme                        *1.0 TB     disk4
   1:                        EFI EFI                     209.7 MB   disk4s1
   2:                 Apple_RAID                         992.2 GB   disk4s2
   3:                 Apple_Boot Boot OS X               134.2 MB   disk4s3
/dev/disk6
   #:                       TYPE NAME                    SIZE       IDENTIFIER
   0:      GUID_partition_scheme                        *1.0 TB     disk6
   1:                        EFI EFI                     209.7 MB   disk6s1
   2:                 Apple_Boot Boot OS X               134.2 MB   disk6s3
/dev/disk7
   #:                       TYPE NAME                    SIZE       IDENTIFIER
   0:      GUID_partition_scheme                        *1.0 TB     disk7
   1:                        EFI EFI                     209.7 MB   disk7s1
   2:                  Apple_HFS Hobbes                  999.9 GB   disk7s2
/dev/disk8
   #:                       TYPE NAME                    SIZE       IDENTIFIER
   0:      GUID_partition_scheme                        *1.0 TB     disk8
   1:                        EFI EFI                     209.7 MB   disk8s1
   2:                 Apple_Boot Boot OS X               134.2 MB   disk8s3
/dev/disk9
   #:                       TYPE NAME                    SIZE       IDENTIFIER
   0:      GUID_partition_scheme                        *2.0 TB     disk9
   1:                        EFI EFI                     209.7 MB   disk9s1
   2:                  Apple_HFS Spaceman Spiff          2.0 TB     disk9s2


Any ideas? I've tried turning off the Energy Saver preference for "shut down hard drives when possible", but it keeps happening.
To troubleshoot this, you could use System Information to inspect the USB tree on all ports and see if you can see the connected hubs.

That might make it easier to diagnose what part of the chain is dropping.

Additionally, the console app can let you search for error messages around the time when the devices go missing. Searching for USB and other disk related messages might turn up some idea if it's an overpower condition where the hardware is shutting down the ports as a preventative measure or due to sustained errors on the USB bus.