NAS RAID Drive – Reclaim Space for Regular Mac Use

I have just upgraded several hard drives on our Synology, and now have 2 bare metal drives of smaller capacity (6TB) that I'd like to reuse on my Mac for other purposes. However I can't seem to get them reformatted for use with the Mac: I am only able to reclaim 1.6TB of the original 6TB (WD Red drive)

Here is how they show up initially:

/dev/disk9 (external, physical):
   #:                       TYPE NAME                    SIZE       IDENTIFIER
   0:      GUID_partition_scheme                        *1.6 TB     disk9
   1:                 Linux_RAID                         2.6 GB     disk9s1
   2:                 Linux_RAID                         2.1 GB     disk9s2

This adds up to about 6TB, and makes sense to see the Linux_RAID partition types given that I pulled these from Synology.

However after reformatting using Disk Utility, I only see a 1.6TB drive, which is confirmed when I do a diskutil list:

/dev/disk9 (external, physical):
   #:                       TYPE NAME                    SIZE       IDENTIFIER
   0:      GUID_partition_scheme                        *1.6 TB     disk9
   1:                        EFI EFI                     209.7 MB   disk9s1
   2:                  Apple_HFS Untitled                1.6 TB     disk9s2

I've tried various combinations of reformatting from the command line, as well [this question] (including diskutil eraseDisk), to no avail. I always seem to end up with a single 1.6TB partition.

Any suggestions on how to fully wipe and reformat the drive to get access to the full capacity?

EDIT: trying to reclaim the partition space did not work either (note that after a reboot, the disk in question is now disk5:

➜  ~ diskutil partitionDisk disk5 1 GPT HFS+ MacintoshHD 100%
Started partitioning on disk5
Unmounting disk
Creating the partition map
Waiting for partitions to activate
Formatting disk5s2 as Mac OS Extended with name MacintoshHD
Initialized /dev/rdisk5s2 as a 1 TB case-insensitive HFS Plus volume
Mounting disk
Finished partitioning on disk5
/dev/disk5 (external, physical):
   #:                       TYPE NAME                    SIZE       IDENTIFIER
   0:      GUID_partition_scheme                        *1.6 TB     disk5
   1:                        EFI EFI                     209.7 MB   disk5s1
   2:                  Apple_HFS MacintoshHD             1.6 TB     disk5s2

EDIT2: getting details on the disk in question:

➜  ~ system_profiler SPStorageDataType | egrep disk5 -B 10 -A 10
    MacintoshHD:

      Free: 1.6 TB (1,602,103,631,872 bytes)
      Capacity: 1.6 TB (1,602,784,641,024 bytes)
      Mount Point: /Volumes/MacintoshHD
      File System: HFS+
      Writable: Yes
      Ignore Ownership: Yes
      BSD Name: disk5s2
      Volume UUID: 96706824-36D6-37F5-BEC0-E8D7BAA06E9B
      Physical Drive:
          Device Name: External
          Media Name: Generic External Media
          Protocol: USB
          Internal: No
          Partition Map Type: GPT (GUID Partition Table)