windows 10 - Accessing "unknown" volume in external drive

I have an external USB drive I use for file backups in my home network (for both Windows and OSX machines). It's normally plugged into my wireless router (TP-Link Archer C9). It works fine but I would like to do some faster file transfers so I plugged it into my Windows 10 machine via USB and Windows won't assign it a drive letter. DISKPART says "type" is "unknown" and, unfortunately, I don't remember what I had formatted it as.

I attempted to follow the answer for this question (How do I mount the EFI partition on Windows 8.1 so that it is readable and writeable?) but DISKPART says there's no volume for me to mount.

DISKPART> list volume

  Volume ###  Ltr  Label        Fs     Type        Size     Status     Info

  ----------  ---  -----------  -----  ----------  -------  ---------  --------
  Volume 0     E                       DVD-ROM         0 B  No Media
  Volume 1         System Rese  NTFS   Partition    100 MB  Healthy    System
  Volume 2     C                NTFS   Partition    930 GB  Healthy    Boot
  Volume 3                      NTFS   Partition    450 MB  Healthy    Hidden
  Volume 4     D   Samsung SSD  NTFS   Partition    232 GB  Healthy
  Volume 5     F   SEAGATE      FAT32  Partition   1863 GB  Healthy
  Volume 6         EFI          FAT32  Partition    200 MB  Healthy    Hidden

DISKPART> list part


  Partition ###  Type              Size     Offset
  -------------  ----------------  -------  -------
  Partition 1    System             200 MB    20 KB
* Partition 2    Unknown            931 GB   200 MB

DISKPART> assign

There is no volume specified.
Please select a volume and try again.

Here's look the Disk Management screen. The highlighted volume is the one I'd like access to. I don't know why I can't see it in DISKPART.