Mac won't access 4 TB Seagate USB drive I don't see it in Finder, I don't see it in Disk Utility. I don't see it in the system profiler. 

diskutil -list
/dev/disk0 (internal, physical):
   #:                       TYPE NAME         SIZE       IDENTIFIER
   0:      GUID_partition_scheme             *1.0 TB     disk0
   1:                        EFI EFI          209.7 MB   disk0s1
   2:                  Apple_HFS Macintosh HD 999.3 GB   disk0s2
   3:                 Apple_Boot Recovery HD  650.0 MB   disk0s3


Tried on two different ports, both of which work with a thumb drive. All ports on this old beast are USB 2.

System: Mac Pro (Early 2008) running 10.11.6 (El Capitan)

Disk: Seagate Expansion 4TB, USB 3

Disk works fine on a Windows laptop (USB 3 port) and a NUC running CoreOS (both USB 2 and USB 3)

USB 3 cable that came with drive in all tests.  

Fsck is not running. 

Console when drive plugged in:


10/22/18 5:25:14.000 PM kernel[0]: USBMSC Identifier (non-unique): 0x00000000 0xbc2 0x231a 0x710, 2
10/22/18 5:25:21.000 PM kernel[0]: USB device 0BC2231AFD100000 - fConsecutiveResetCount = 1.
Short version: I have an old Mac with old-spec USB ports that only supply a limited amount of power to the connected device. That's enough to drive the circuitry (hence the console report) but not enough to spin up the physical disk. Of course, this is not a problem with thumb drives.

Solution: connect the drive through a powered hub, to provide the extra juice that's not coming from the Mac. It's possible that an older powered hub wouldn't work. Fortunately, current models are quite cheap.

This is arguably a duplicate of this question, where the problem is the same. But it's an old question, a bit of a mess, and the best solution offered (use a special cable to feed extra power) is not practical for most people.