I have no idea what's causing this, but I'm going to start by guessing that it's some udev thing controlled by systemd.  Corrections most welcome.

Insert a USB flash drive, and use whatever you like (I used gparted) to put an MBR on it and define a partition, say /dev/sdb1, and format it as ext4.

Leave the drive inserted, and reboot the system.  When it comes up, log into a KDE desktop.  The Device Manager will produce a popup showing the removable drive partition, but a "df" will show that it is not mounted.  Likewise, "umount /dev/sdb1" will say that /dev/sdb1 is not mounted.

Now try "mount -t ext4 /dev/sdb1 /mnt/disk", and you'll be told that /dev/sdb1 is already mounted or /mnt/disk is busy.  If you create a totally new mount point, say /mnt/disk2, and retry the mount using that, you'll get the same response, indicating that the problem is actually that /dev/sdb1 is considered to be in use.

This is consistently reproducible, but it gets better.  Now, edit /etc/fstab and add an entry mounting /dev/sdb1 at /mnt/disk, and reboot.

"df" will show that /dev/sdb1 is indeed now mounted at /mnt/disk, and is perfectly usable.

My experience with this stuff dates from the days when the error messages actually meant what they said, and I know of nothing other than a mount that would mark /dev/sdb1 as "in use".  But clearly something during system initialization is grabbing /dev/sdb1 in some way, and just as clearly the systemd unit that mounts the /etc/fstab mounts gets there first and prevents it.

By way of background, this came up because I was trying to use drakx-in-chroot to install a system on a USB Flash Drive partition.  The script I used for this would normally umount the partition, create a filesystem on it, mount it under /mnt/disk, and then run drakx-in-chroot.  It was failing on the mkfs.ext4, saying that the partition was in use and that it would not create a filesystem there, and then again on the mount.  When I added the partition to /etc/fstab, rebooted, and issued the drakx-in-chroot command directly, it worked fine (note that the partition had already been formatted as ext4 by gparted above, so the preliminary steps in the script were not necessary).

Reproducible: 

Steps to Reproduce: