mount -a failed

$ mount -a
output /dev/sdc1 already mounted or /dir busy

originally had four hard disks, but later removed one /dev/sdc, and /dev/sdd is still there.
After restarting, because the original /dev/sdc is gone, the previous /dev/sdd becomes /dev/sdc, and the previous /dev/sdd1 /dir ext4 default 1 2 in /etc/fstab cannot be found When it reaches /dev/sdd1, there is a problem with startup. Press -S to skip scanning /dev/sdd before restarting.
The directory mounted on /dev/sdd1 before reboot is not mounted.

solution
Change /dev/sdd1 in /etc/fstab to /dev/sdc1 and restart it and it will be ok.
The drive letter may be different in different situations, and the specific situation is analyzed in detail.