If I’m not mistaken, the encryption key is only stored in the device in which the SD card was encrypted (in a file in /data/misc/vold). If that is true, and there is no backup of that folder that you can access, then that’s bad news for being able to recover data from an Android encrypted SD card.
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As for block devices: in Linux devices show up as files in the /dev/ folder. Storage devices often start with sd and then a letter; for example the main storage device is usually /dev/sda. You’ll just need to figure out which one is the one for your sd card. In a terminal, running dmesg gets you recent system messages, if you do that just after inserting the sd card, you often get hint as to which sdx device it is. You can then check which partitions are on the device using fdisk -l /dev/sdx, where sdx should be replaced by the relevant device name. You may need to run this and other commands as root (if you get access denied errors) - this can be done by adding sudo to the beginning of the command. If you get something like this (source):
#Start End Size Type Name 1 2048 34815 16M unknown android_meta 2 34816 15257566 7.3G unknown android_expand
(i.e. two partitions of unknown type, labelled android_meta and android_expand) then that’s a strong indication you have an encrypted sd card.
If you do want a copy of the block device, a popular way of doing so is dd if=/dev/sdx of=~/image.img. You can then run commands on the image file you created in the same way as if you were using the actual device. (e.g. fdisk -l ~\image.img should give you the same results).
If you’re more comfortable in Windows, right click computer in the start menu (or in the file explorer) and choose ‘manage’. In the window that pops up, you should be able to see partition layouts under Storage > Disk management. If you see two partitions on the SD card, one named android_meta and the other named android_expand, the SD card is probably encrypted.