You could also do it another way - find out the uuid of the disk by issuing the command when that usb is inserted first time (the objective is to find out the uuid) by using vol_id
NAME
vol_id - probe filesystem type and read label and uuid
SYNOPSIS
vol_id [--export] [--type] [--label] [--label-raw] [--uuid]
[--skip-raid] [--probe-all]
[--offset=bytes] [--debug] [--help]
[device]
DESCRIPTION
vol_id is usually called from a udev rule, to provide udev with the
filesystem type, the label and the
uuid of a volume. It supports most of
the common
filesystem formats and detects various raid setups to prevent the
recognition of raid members as a
volume with a filesystem.
OPTIONS
--export
Print all values in key/value format to import them into
the environment.
--type
Print the filesystem type.
--label
Print the safe version of volume label suitable for use as
filename.
--label-raw
Print the raw volume label.
--uuid
Print the uuid of a volume.
--skip-raid
Skip detection of raid metadata.
--probe-all
Probe for all types and print all matches.
--offset=bytes
Start probing at the given offset, instead of the beginning of
the volume. The offset value is
specified in bytes.
--debug
Print debug messages to stderr.
--help
Print usage.
ENVIRONMENT
UDEV_LOG
Set the syslog priority.
EXIT STATUS
vol_id will only return successful if the value asked for is
not empty. All trailing whitespace
will be removed, spaces replaced by
underscore and slashes
ignored.
Then it's a matter of checking the vol_id once you have the value for that disk...