Answer by Thomas for SD card showing up in `dmesg` but not elsewhere

The problem seems to be related to a misbehaving auto suspend function of the ums_realtek driver.

From the output of dmesg of the question one can see that the device gets detached again 4 seconds after the SD card is assigned a device name.

[  210.871920] sd 3:0:0:0: [sdc] Attached SCSI removable disk[  214.498456] usb 1-6: USB disconnect, device number 5

A simple fix during runtime is to disable the auto suspend function of the ums_realtek driver. Not sure if both are needed or if just one does the trick. Maybe @MaciekS can do further testing on this.

echo 0 | sudo tee /sys/module/ums_realtek/parameters/ss_en  echo 0 | sudo tee /sys/module/ums_realtek/parameters/auto_delink_en

To make these settings reboot resistant, one has to create a file.

# sudo su# cat > /etc/modprobe.d/ums_realtek.conf << 'UMS'options ums_realtek ss_en=0options ums_realtek auto_delink_en=0UMS

Since I do not have a Realtek based SD card reader nor a Ubuntu 19.04 I cannot further debug this.
It also might be that a user space program or daemon like power saving things is triggering this and the problem is solved by removing the possibility to suspend USB devices.