Mint USB An operation is already pending
 

This started happening, I think, after an upgrade.

If I reboot, login and insert a USB stick it does not mount although it used to.
Using the file manager the USB appears in "Computer" but clicking on the USB entry produces a window alert "An operation is already pending"

The aberant USB works on other linux machines and other USBs that work elsewhere also fail to work as well so this is an OS problem.

/dev/sdb1: LABEL="USB DISK" UUID="79D0-5F8F" TYPE="vfat" PARTUUID="797cd3d3-01"

Can mount (as root) using a mount command
dmesg shows this;
[11321.824899] scsi host7: usb-storage 3-2:1.0
[11322.846085] scsi 7:0:0:0: Direct-Access USB DISK 2.0 PMAP PQ: 0 ANSI: 6
[11322.846308] sd 7:0:0:0: Attached scsi generic sg2 type 0
[11323.480419] sd 7:0:0:0: [sdb] 15122432 512-byte logical blocks: (7.74 GB/7.21 GiB)
[11323.481249] sd 7:0:0:0: [sdb] Write Protect is off
[11323.481251] sd 7:0:0:0: [sdb] Mode Sense: 23 00 00 00
[11323.481999] sd 7:0:0:0: [sdb] No Caching mode page found
[11323.482005] sd 7:0:0:0: [sdb] Assuming drive cache: write through
[11323.485840] sdb: sdb1
[11323.488377] sd 7:0:0:0: [sdb] Attached SCSI removable disk

Forums show this is a common problem for Mint and Ubuntu but solutions mostly seem to involve the problem suddenly disappearing. One solution proposes disabling automount. Which I would rather not do.

Anyone else encountered this?

It might also be a machine problem. I assume you've tried using all the different USB ports on the machine, so your next move may be to try booting off a live install media (preferably DVD) of a fresh copy of the same distro and see if that subsequently successfully mounts the USB device when inserted.

USB drives
 

Did check all of the USB inputs and none work. However. I tried a fresh reboot logged in, then USB suddenly worked. I correctly unmounted but some time later (starting to think all had fixed itself) it started hfiling to mount again with the "operation pending error". Its as though something gets mounted but fails to unmount but even if a USB is not unmounted the OS should still mount new USB drives.

The USB keyboard and mouse works.

I can try booting a dist CD but I imagine that a new OS will work. I don't really want to do a fresh install. The problem is to find how to fix the current OS.

Sorry if I didn't get my meaning across correctly. I didn't intend that you do a fresh install, just try to mount your USB device using a live version of your distro - if it still doesn't work then the machine *may* be at fault rather than the distro.

please have a good look at this search and specify which situation applies to you best.

Searching
 

Yes I have looked carefully at such searches previously. After not finding a satisfactory answer I come to linuxquestions.

Some solutions require disabling automount and sometimes referring to an internal hard drive rather than a USB mount. Disabling auto-mount will mean any use will have to manually mount a USB or drive and then they'll start saying "this is no problem in Windows"

This problem seem related to a Mint upgrade and possibly a switch by the OS maintainers to another USB handling system.

I get that too sometimes, when I have a fm open plug in then click on it before the fm has a chance to fully mount it, it gives me that message. I'm working on it yo!
I just wait a moment and no worries, it's mounted.

Cludge Solution
 

The file /var/syslog complains the USB does not have a label - although it does.
It can't deduce a mount point, presumably using the disk label.

I manually mounted the USB
By some miracle I tried putting in a second USB that normally fails and to my surprise it automounts.
The solution then is to leave a dummy USB in place although i found you can pull it out and it doesn't unmount and then any other USBs do mount okay.

My OS is
NAME="Linux Mint"
VERSION="19 (Tara)"
but other forum groups indicate this problem is common and comes about after an upgrade.
Hopefully Mint will bring out an upgrade that will cure the problem but for the meantime this cludge will do

people have reported problems when manually mounting into one of the directories reserved by whichever daemon takes care of these things.
in other words: don't mess around in /media, /run/media etc.

I'm sorry, but this isn't a solution. It's a workaround. I've been having this same problem for some time, and it's very frustrating that it hasn't been fixed.

Work Around
 

I completely agree with farside268.

My machine is a server/desktop and I have naive users plugging in USBs for system processing.
Expecting them to su to root and do a mount is not on. I have set up Filezilla on their Windows desktops and an ftp server on the server.

Curiously I have other Mint installations on other machines and the auto USB mount is okay.
I could do a new system install but the server is growing in importance and a radical system change would be time consuming and interrupt services. Hopefully someone will identify something I have missed.