Attached external drive unmountable

When I attach a drive to ubuntu 18.04 it is not listed in /dev.

ls -al /dev | grep sd

returns only sda (boot drive)

If I list disks, I can see the device (full fdisk response below - I'm no longer convinced the partition error is referring to the attached drive). But not the disk.

dmsg doesn't show any errors when i plug the device in either:

[10134.191871] usb 2-3: USB disconnect, device number 2
[11563.988155] usb 2-3: new high-speed USB device number 6 using xhci_hcd
[11564.138381] usb 2-3: New USB device found, idVendor=0bc2, idProduct=ac19
[11564.138387] usb 2-3: New USB device strings: Mfr=2, Product=3, SerialNumber=1
[11564.138390] usb 2-3: Product: Device Name
[11564.138393] usb 2-3: Manufacturer: Seagate
[11564.138395] usb 2-3: SerialNumber: 0000000000000000

So i guess the questions are:

1. why can't i mount this device as is?
2. how do i get this device to be mountable?

ps

I now have two devices that mount happily on other machines, mac and windows, but fail to mount on this ubuntu box.

user@box:~$ sudo fdisk -l
Disk /dev/sda: 465.8 GiB, 500107862016 bytes, 976773168 sectors
Units: sectors of 1 * 512 = 512 bytes
Sector size (logical/physical): 512 bytes / 4096 bytes
I/O size (minimum/optimal): 4096 bytes / 4096 bytes
Disklabel type: dos
Disk identifier: 0x26464355

Device     Boot   Start       End   Sectors   Size Id Type
/dev/sda1  *       2048    999423    997376   487M 83 Linux
/dev/sda2       1001470 976771071 975769602 465.3G  5 Extended
/dev/sda5       1001472 976771071 975769600 465.3G 8e Linux LVM

Partition 2 does not start on physical sector boundary.




Disk /dev/mapper/box--vg-root: 461.4 GiB, 495418605568 bytes, 967614464 sectors
Units: sectors of 1 * 512 = 512 bytes
Sector size (logical/physical): 512 bytes / 4096 bytes
I/O size (minimum/optimal): 4096 bytes / 4096 bytes


Disk /dev/mapper/box--vg-swap_1: 3.9 GiB, 4173332480 bytes, 8151040 sectors
Units: sectors of 1 * 512 = 512 bytes
Sector size (logical/physical): 512 bytes / 4096 bytes
I/O size (minimum/optimal): 4096 bytes / 4096 bytes
Disklabel type: gpt
Disk identifier: B09AD881-9F34-401E-9A48-74B04F72C504

Device                           Start     End Sectors  Size Type
/dev/mapper/box--vg-swap_1p1  4096 8148991 8144896  3.9G Linux filesystem