Unable to use disk removed from Synology NAS with a Mac - wrong partition size showing I have 2 x 3TB disks I've removed from a Synology NAS. When I try to mount these disks on OS X, they show as only having a single 800GB partition.

I've tried erasing in the Disk Util app, but it only finds and creates a new partition for the 800GB.

Repartitioning, erasing, repairing, all only see the disk as 800GB.

Any idea how to get these back to 3TB each? Both disks behave identically, and are WD RED disks WD30EFRX

disktutil - List...

/dev/disk3 (external, physical):
   #:                       TYPE NAME                    SIZE       IDENTIFIER
   0:      GUID_partition_scheme                        *801.6 GB   disk3
   1:                        EFI EFI                     209.7 MB   disk3s1
   2:                  Apple_HFS Untitled                801.2 GB   disk3s2


And the info...

DT:~ david$ diskutil info /dev/disk3
   Device Identifier:         disk3
   Device Node:               /dev/disk3
   Whole:                     Yes
   Part of Whole:             disk3
   Device / Media Name:       EFRX-68EUZN0

   Volume Name:               Not applicable (no file system)
   Mounted:                   Not applicable (no file system)
   File System:               None

   Content (IOContent):       GUID_partition_scheme
   OS Can Be Installed:       No
   Media Type:                Generic
   Protocol:                  USB
   SMART Status:              Not Supported

   Disk Size:                 801.6 GB (801569726464 Bytes) (exactly 1565565872 512-Byte-Units)
   Device Block Size:         512 Bytes

   Read-Only Media:           No
   Read-Only Volume:          Not applicable (no file system)

   Device Location:           External
   Removable Media:           Fixed

   Virtual:                   No


What avenue to wipe this drive and set it up for new filesystem to be installed exist?
This usually happens for one of two reasons: Either your partition table is incorrect/corrupted, or you have a limiting disk interface converter connecting.

Corrupted partition table

You'll need to wipe the partition table and write a new one, which will then allow you to use the full disk.

Follow the instructions below the heading "Erase and repartition a device" on this link to do so:

https://support.apple.com/kb/ph22240?locale=en_US

If you want to be absolutely sure that you have wiped the partition table, then choose to make it an MBR table first, and then do it again and choose GUID the second time.

Limiting disk interface converter

Some old USB to SATA adapters actually limit what drive size they will report over USB. This is because they convey the number of sectors on the drive as a 32-bit number only. Your disk (3 TB) requires 36-bits to describe the number of sectors. This would make the drive report as approx. 750-800 GB instead of the correct size.

You'll need to use a different adapter to connect the disk to solve this.