I started using Ubuntu(18.04) recently and to this moment I am very satisfied.
I am facing this problem with the Kindle Paperwhite not being recognized by the usb.
I have tested other devices and worked normally, only the Kindle is not recognized. it even turns on the charging light near the Power button.
I ran some commands in the terminal to try to identify if Ubuntu is recognizing the device and apparently this is not happening.
Among these commands one of them was ls /dev/ | grep sd
The output of the command was the same whether the device was connected or disconnected.
I tested it on other USB ports.
OUTPUTS with Kindle connected
$ sudo apt list *mtp* | grep installed
WARNING: apt does not have a stable CLI interface. Use with caution in scripts.
gmtp/bionic,now 1.3.10-1 amd64 [installed]
libmtp-common/bionic,bionic,now 1.1.13-1 all [installed]
libmtp-runtime/bionic,now 1.1.13-1 amd64 [installed]
libmtp9/bionic,now 1.1.13-1 amd64 [installed]
libnet-smtp-ssl-perl/bionic,bionic,now 1.04-1 all [installed]
mtp-tools/bionic,now 1.1.13-1 amd64 [installed]
$ lsusb
Bus 001 Device 005: ID 0bda:0129 Realtek Semiconductor Corp. RTS5129 Card Reader Controller
Bus 001 Device 008: ID 0cf3:0036 Atheros Communications, Inc.
Bus 001 Device 003: ID 0bda:5756 Realtek Semiconductor Corp.
Bus 001 Device 002: ID 8087:8000 Intel Corp.
Bus 001 Device 001: ID 1d6b:0002 Linux Foundation 2.0 root hub
Bus 003 Device 001: ID 1d6b:0003 Linux Foundation 3.0 root hub
Bus 002 Device 002: ID 046d:c07f Logitech, Inc.
Bus 002 Device 001: ID 1d6b:0002 Linux Foundation 2.0 root hub
$ lsblk
NAME MAJ:MIN RM SIZE RO TYPE MOUNTPOINT
loop0 7:0 0 34,8M 1 loop /snap/gtk-common-themes/1122
loop1 7:1 0 140,7M 1 loop /snap/gnome-3-26-1604/78
loop2 7:2 0 14,5M 1 loop /snap/gnome-logs/45
loop3 7:3 0 128,5M 1 loop /snap/vscode/77
loop4 7:4 0 91M 1 loop /snap/core/6405
loop5 7:5 0 2,3M 1 loop /snap/gnome-calculator/260
loop6 7:6 0 140,7M 1 loop /snap/gnome-3-26-1604/74
loop7 7:7 0 7,5M 1 loop /snap/canonical-livepatch/58
loop8 7:8 0 13M 1 loop /snap/gnome-characters/139
loop9 7:9 0 34,6M 1 loop /snap/gtk-common-themes/818
loop10 7:10 0 53,7M 1 loop /snap/core18/719
loop11 7:11 0 3,7M 1 loop /snap/gnome-system-monitor/57
loop12 7:12 0 174M 1 loop /snap/spotify/34
loop13 7:13 0 5,8M 1 loop /snap/tor/2
loop14 7:14 0 66M 1 loop /snap/discord/91
loop15 7:15 0 91M 1 loop /snap/core/6350
loop16 7:16 0 7,5M 1 loop /snap/canonical-livepatch/54
sda 8:0 0 223,6G 0 disk
├─sda1 8:1 0 512M 0 part /boot/efi
├─sda2 8:2 0 732M 0 part /boot
└─sda3 8:3 0 222,4G 0 part
└─sda3_crypt 253:0 0 222,4G 0 crypt
├─ubuntu--vg-root 253:1 0 221,4G 0 lvm /
└─ubuntu--vg-swap_1 253:2 0 976M 0 lvm [SWAP]
sdb 8:16 0 931,5G 0 disk
└─sdb1 8:17 0 931,5G 0 part /media/pedro/Vault
$ jmtpfs
No mtp devices found.
$ dmesg | curl -F c=@- https://ptpb.pw
date: 2019-02-28T19:33:00.232964+00:00
digest: b6f4b200bfc91f41fdfb31e04f100fed679714bf
long: ALb0sgC_yR9B_fsx4E8QD-1nlxS_
short: lxS_
size: 61329
status: created
url: https://ptpb.pw/lxS_
uuid: f104d68c-0a1c-48cc-9817-05146950ba14
It was a problem with the JBL USB cable. From what I realized this cable only serves to recharge devices and not to transfer files.
From my experience, it can even be the connector which is flawed : tweaking it a bit inside the kindle slot mounted it.