@JMac ok bigger picture, in Linux
hardware devices are named. see the output of ls /dev
disk devices are usually named sd??? where xxx is a letter, and partition number
letter a is the first device b the second etc
sometime in the past the type of storage device was also used as part of the name, scsi, atapi …
for this case they are named sd (storage device)
sda is the first storage device
sda1 is the first partition on the sda device
on those partitions are a logical way of storing data. most linux and all windows devices store file data in sectors, 512 byte chunks called sectors. and then the file system (way of organizing those sectors)
applies some data structure on top. it’s a directory or a file. tables in other sectors build trees of data to describe the entire partition.
there are different layouts depending on vendor and intended use
FAT, exFAT, NTFS EXT3, EXT4, and a host of others.
all is wonderful until some sector or more gets damaged… machine was powered off during write, a hardware failure…
now the filesystem code is confused… says read sector 853, and the bits there will tell it where the next sector is for this file. but the bits don’t point to the right place… Oops
some file systems include a duplicate set of bits do there is an alternative way . some use them only for recovery.
sd card hardware is known for being fragile. it was designed for lots of reads,with few writes. camera picture music file. NOT an os that is waiting logs and other stuff constantly.
anyhow.
to check and correct these kinds of problems with the Linux ext file system we need to run thr fsck program on the raw partition. and make sure that moone else is using it.
unmount takes it out of circulation. no users files open
fsck and e2fsck need to read the raw sectors on the partition, but we just unmounted it.
so we need to provide the hardware name for the program to use.