Comment 17 for bug 1773597

I booted another machine (currently running 18.04) from the USB memory stick with 16.04 on it. I plugged in three USB memory sticks. Each came up in a couple of seconds. When that machine is running 18.04 it takes 4+ minutes for a single memory stick to be available.

Note that dmesg shows that the OS sees the memory stick nearly immediately. It's Nautilus (guessing) that doesn't make it available.