mjhjmtu

I have a block device. It is removed from the system and later added again. How can I verify that it is exactly the same device?

The device will always be a mass storage device (which I'm not planning to write to and will only mount read-only).

I could md5sum both devices and verify that the hashes match, but this is time consuming for large storage devices.

Is there a quicker method?

The output of fdisk -l $device includes a line starting 'Disk identifier:' with some detail from the device that is usually unique and persistent for it.
Illustration:

# fdisk -l /dev/sda | grep 'Disk identifier:'

You could use the devices UUID.
lsblk -f or blkid .

UUID can be set (with tune2fs) but for most use cases it should be unique enough.

Hardcoded unique(?) ID

Most mass storage devices (maybe not all) have a hardcoded unique ID, that is seen at /dev/disk/by-id. The following command line will show this ID,

sudo ls -l /dev/disk/by-id|grep -v '[1-9]$'|tr -s ' ' 't'|cut -f 9- | sed 's#../../#/dev/#'

and also show which block device it is represented by, '-> /dev/sdx'

Example (my working computer with three USB pendrives connected),

$ sudo ls -l /dev/disk/by-id|grep -v '[1-9]$'|tr -s ' ' 't'|cut -f 9- | sed 's#../../#/dev/#'

ata-HL-DT-STDVD+-RW_GSA-H21L -> /dev/sr0
ata-KINGSTON_SKC300S37A120G_50026B724703BCA8 -> /dev/sda
ata-WDC_WD4002FYYZ-01B7CB1_K3GWHAEB -> /dev/sdb
usb-Corsair_Voyager_GT_3.0_070883862E1B9719-0:0 -> /dev/sdc
usb-SanDisk_Cruzer_Blade_200429068118E7C2CFFD-0:0 -> /dev/sde
usb-SanDisk_Cruzer_Blade_200429068118F440A09E-0:0 -> /dev/sdd
wwn-0x5000cca25ccc7f97 -> /dev/sdb
wwn-0x50026b724703bca8 -> /dev/sda

UUID of file system(s)

If there are partitions with file systems, there are also UUIDs for each file system, and these are often but not always unique, depending on if they were created individually or cloned from some common original file system or image file. The UUIDs can be shown by blkid and by lsblkvia the following command line,

lsblk -l -o name,size,UUID,model

Example (my working computer with three USB pendrives connected),

$ lsblk -l -o name,size,UUID,model
NAME SIZE UUID MODEL
sda 111,8G KINGSTON SKC300S
sda1 106,9G 0ac1cb43-1609-4fc3-8c69-3e21299729bc 
sda2 5G 6d54c49d-31ac-45fe-917c-2335bcfe7399 
sdb 3,7T WDC WD4002FYYZ-0
sdb1 510M 9F05-5B18 
sdb2 5G 08b7164f-8852-451b-9624-63b16a66359a 
sdb3 10G fa242ddd-90f8-4603-af5c-c89f4b71ac70 
sdb4 15G 491a6a2b-4867-44d5-94d8-082f79066a5a 
sdb5 1M 
sdb6 100G 44156f43-0958-4ea1-800d-b02afbc7d306 
sdb7 3,5T 862210fd-a6fd-4fe3-913c-e18e1448ef36 
sdb8 5G 6958d86a-57a9-4e40-8376-1e41258e5810 
sdc 29,5G Voyager GT 3.0 
sdc1 9,1G 38028BF9184E3FB4 
sdc2 1M 
sdc3 244M 4299-B748 
sdc4 1,8G 2018-04-26-18-43-51-00 
sdc5 18,4G 033ffb75-518d-4335-89cb-ef7d159cf20b 
sdd 3,7G 2016-08-24-07-06-03-00 Cruzer Blade 
sdd1 1,3G 2016-08-24-07-06-03-00 
sde 3,7G 2016-07-19-21-27-51-00 Cruzer Blade 
sde1 1,4G 2016-07-19-21-27-51-00 
sde2 2,3M 0F7B-9366 
sr0 1024M DVD+-RW GSA-H21L

These methods are enough to help me identify the connected mass storage devices after a quick manual inspection.

It is possible to use the hardcoded ID and/or the UUIDs automatically. For example, you can create a corresponding md5sum for every mass storage device that you have, and store the md5sums in variables in a shellscript, where you can check for matches.