Unmount
You can unmount all partitions without trying to unmount the device itself with
sudo umount /dev/sdx?*
Replace the device letter x
with the relevant device letter, for example b
Clone
It is a good idea to use a cloning tool with a final checkpoint,
Accidentally did dd /dev/sda
Scroll down to 'Safer tools to create USB boot drives with Ubuntu'
Sync, Partprobe, Mount
sync
sudo partprobe
Try mounting after creating the new system on the USB stick
This command line should work with one or more partitions,
for i in $(lsblk -l -o name|grep -E "sdx.{1,}");do sudo mkdir -p /media/$USER/"$i";sudo mount /dev/"$i" /media/$USER/"$i";done
Replace the device letter x
with the relevant device letter, for example b