I mounted a flash drive, but under "devices" in deb9 it does not show up. Also, it is mounted as "mount /dev/sda1 /media" and it does not show up in the media folder either. It is "ext4" formatted.

It is mounted as df -h shows.

command-line mounted drives won't show up in the file manager.
try to unmount it, replug it, and then simply use the file manager to do the mounting.

also, why did you add 'chromium' to thread title?

Thanks. How would I mount it using the file manager? It does show up there now, but I can not add anything to it. The only thing that shows are 2 folders with "X"s on them. And I can not copy and paste into the directory or whatever is holding them. I get the feeling that it "auto-mounted" since right clicking gives the option to "unmount".

On the other hand, if I did use the command line, how would I access it then, though I get the feeling I would need to be more familiar with command-line commands.....?

I wouldn't worry about it. The file manager will frequently not show certain mounted volumes. It's all a matter of how it's programmed and configured to work. It doesn't mean the volume isn't mounted.

Either way, I need to copy and paste files to it and I am not able to do that...

Please paste the output from the following command and indicate which is your flash drive:

Thanks. Can you clarify the quotes part....what is suppose to go in there or am I suppose to paste it into the terminal exactly as you have it written?

Exactly as written.

MAYBE THIS IS WHAT YOU NEED: (***sda1 = exthdd)

lsblk
NAME MAJ:MIN RM SIZE RO TYPE MOUNTPOINT
sda 8:0 0 1.8T 0 disk
└─sda1 8:1 0 1.8T 0 part
mmcblk0 179:0 0 14.9G 0 disk
└─mmcblk0p1 179:1 0 14.7G 0 part /
mmcblk2 179:8 0 14.6G 0 disk
├─mmcblk2p1 179:9 0 12.2G 0 part
├─mmcblk2p2 179:10 0 16M 0 part
├─mmcblk2p3 179:11 0 1K 0 part
├─mmcblk2p5 179:13 0 16M 0 part
├─mmcblk2p6 179:14 0 16M 0 part
├─mmcblk2p7 179:15 0 768M 0 part
├─mmcblk2p8 259:0 0 16M 0 part
├─mmcblk2p9 259:1 0 32M 0 part
├─mmcblk2p10 259:2 0 768M 0 part
├─mmcblk2p11 259:3 0 16M 0 part
├─mmcblk2p12 259:4 0 16M 0 part
├─mmcblk2p13 259:5 0 16M 0 part
├─mmcblk2p14 259:6 0 32M 0 part
├─mmcblk2p15 259:7 0 16M 0 part
└─mmcblk2p16 259:8 0 640M 0 part
mmcblk2boot0 179:16 0 4M 1 disk
mmcblk2boot1 179:24 0 4M 1 disk

==================================================================================================== =======
df -h
Filesystem Size Used Avail Use% Mounted on
udev 998M 0 998M 0% /dev
tmpfs 202M 17M 185M 9% /run
/dev/mmcblk0p1 15G 14G 1013M 94% /
tmpfs 1009M 0 1009M 0% /dev/shm
tmpfs 5.0M 4.0K 5.0M 1% /run/lock
tmpfs 1009M 0 1009M 0% /sys/fs/cgroup
tmpfs 1009M 12K 1009M 1% /tmp
tmpfs 202M 0 202M 0% /run/user/1000
/dev/sda1 1.8T 1.4T 371G 79% /exthdd

Are you having difficulty cutting and pasting the command I gave you?

mount | grep "^/dev/"

/dev/mmcblk0p1 on / type ext4 (rw,noatime,nodiratime,errors=remount-ro,commit=600)
/dev/sda1 on /exthdd type ext4 (ro,noexec,relatime,commit=15,data=ordered)

Based on this, /dev/sda1 is mounted at /exthdd.

It would be better to have run the command cited by Hydrurga. (Which between my starting this reply and now, I see that you have - and also re-affirming my opinion that /dev/sda1 is mounted at /exthdd.)

How sure are you that this drive is /dev/sda1? Suggest you look at the directory under /exthdd and see if the files there are what you expect.This statement appears inaccurate. Mounted file systems are visible from a file manager. When something auto-mounts a file manager window may pop-up to show it, however sometimes do not.

Meanwhile if you mount something using the command line and subsequently open a file manager window in a GUI you will be able to see all mounted file systems.

I did run the command cited by H...it is 2 messages up, the output, that is.
How sure am I that this is sda1? It is definitely the only "1.8T" drive connected to my computer.

How do I find the directory under exthdd? Under devices in file manager, there is "system file". I can get to "media" folder there, but there is nothing about 'exthdd'.

Thanks.

There's your problem - the disk is being mounted as read-only. Why, I don't know. You could try force mounting it as read-write but, being the cautious guy I am, I would first check the disk first with fsck.

To do that, unmount the disk and then check it with fsck -n /dev/sda1.

I purposely mounted it as read-only so if there is any malware on my computer, it will not transfer over to the my exthdd.

Don't know if this helps....but when I try to unmount with: umount /dev/sda1 /exthdd, it says that the "exthdd" is not mounted, even though the 'df-h' command "mounted" says that it is mounted on the exthdd.

I'll run the disk check now.