[solution] how to mount external hard drive after reinstalling Ubuntu system

Note, your external hard drives are still there, you only need to mount them properly.
For example, in my case, I have two external hard drives, one has 4T space and antoher have 2T space. I want to name them as 4T and 2Tssd, respectively.
Here is how:

#step1: check the disk information, pay attention to the external drive:
sudo fdisk -l
#Your screen display lots of information, and you should find something like:
...
Disk /dev/sda: 3.7 TiB, 4000787030016 bytes, 7814037168 sectors
Units: sectors of 1 * 512 = 512 bytes
Sector size (logical/physical): 512 bytes / 4096 bytes
I/O size (minimum/optimal): 4096 bytes / 4096 bytes
Disklabel type: gpt
Disk identifier: 6047D23B-339C-4AE5-90C5-C0DD7F75FE22

Device     Start        End    Sectors  Size Type
/dev/sda1   2048 7814035455 7814033408  3.7T Linux filesystem


Disk /dev/sdb: 1.9 TiB, 2048408248320 bytes, 4000797360 sectors
Units: sectors of 1 * 512 = 512 bytes
Sector size (logical/physical): 512 bytes / 512 bytes
I/O size (minimum/optimal): 512 bytes / 512 bytes
Disklabel type: dos
Disk identifier: 0x93a92f38

Device     Boot Start        End    Sectors  Size Id Type
/dev/sdb1        2048 4000797359 4000795312  1.9T 83 Linux

#step2, prepare mounting poing
sudo mkdir -p /media/2Tssd
sudo mkdir -p /media/4T

#step3: mount
sudo mount /dev/sdb1 /media/2Tssd
sudo mount /dev/sda1 /media/4T