Cannot find mounted disk : "already mounted or mount point busy."

Im struggling to find why I cannot access a seagate drive.
System says it’s mounted:

me@helios64:~$ sudo mount /dev/sdb1 /mnt/disk2/
mount: /mnt/disk2: /dev/sdb1 already mounted or mount point busy.

But cannot umount it:

me@helios64:~$ sudo umount -f /dev/sdb
umount: /dev/sdb: not mounted.

And also, it is not present in /etc/mtab:

me@helios64:~$ cat /etc/mtab | grep sd
cgroup2 /sys/fs/cgroup/unified cgroup2 rw,nosuid,nodev,noexec,relatime,nsdelegate 0 0
nfsd /proc/fs/nfsd nfsd rw,relatime 0 0
/dev/sda1 /srv/dev-disk-by-label-uno ext4 rw,relatime,jqfmt=vfsv0,usrjquota=aquota.user,grpjquota=aquota.group 0 0
/dev/sda1 /export/PLEX ext4 rw,relatime,jqfmt=vfsv0,usrjquota=aquota.user,grpjquota=aquota.group 0 0

Only my first drive sda is showing

me@helios64:~$ dmesg | grep sdb
[    5.014287] sd 1:0:0:0: [sdb] 976773168 512-byte logical blocks: (500 GB/466 GiB)
[    5.014358] sd 1:0:0:0: [sdb] Write Protect is off
[    5.014373] sd 1:0:0:0: [sdb] Mode Sense: 00 3a 00 00
[    5.014487] sd 1:0:0:0: [sdb] Write cache: enabled, read cache: enabled, doesn't support DPO or FUA
[    5.041141] sd 1:0:0:0: [sdb] Attached SCSI disk

Tried fsck’ing the drive, but it won’t let me… as it says is in use

me@helios64:~$ sudo  fsck -f /dev/sdb
fsck from util-linux 2.33.1
e2fsck 1.44.5 (15-Dec-2018)
/dev/sdb is in use.
e2fsck: Cannot continue, aborting.

fdisk -l output:

me@helios64:~$ sudo fdisk -l
[...]
Disk /dev/sdb: 465,8 GiB, 500107862016 bytes, 976773168 sectors
Disk model: ST3500820AS     
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: gpt
Disk identifier: 91A91874-7314-4C83-8061-0904AE8BF0B1

Device     Start       End   Sectors   Size Type
/dev/sdb1   2048 976773134 976771087 465,8G Linux filesystem

Any ideas how to

• Force mount, umount?
• Force fsck, as I doubt on the drive’s integrity
• Find out where is it mounted? (even I don’t think so)

Thank you for your help

EDIT: Tried with the –bind option, but it answers:

mount(2) system call failed: not a directory

Dont understand…

EDIT 2: After mounting successfully, now I cannot write to it… dmesg shows a myriad of:

[ 7822.964530] EXT4-fs (sdb1): I/O error while writing superblock
[ 7822.964543] EXT4-fs error (device sdb1) in ext4_convert_unwritten_io_end_vec:4798: Journal has aborted
[ 7822.964605] EXT4-fs (sdb1): I/O error while writing superblock
[ 7822.964612] EXT4-fs (sdb1): failed to convert unwritten extents to written extents -- potential data loss!  (inode 12582922, error -30)

and then:

[ 7822.965799] JBD2: Error -5 detected when updating journal superblock for sdb1-8.
[ 7822.972166] EXT4-fs (sdb1): I/O error while writing superblock
[ 7822.972182] EXT4-fs error (device sdb1): ext4_journal_check_start:83: Detected aborted journal
[ 7822.972187] EXT4-fs (sdb1): Remounting filesystem read-only
[ 9018.053019] scsi_io_completion_action: 25 callbacks suppressed

Does this mean I can finally throw away the drive?

EDIT 3:

Oh! there’s this big known problem

Does anybody know if I can update the firmware from command line, or USB external? I have this external box for SATA/USB connection.

[Armbian Debian buster 10 Helios64 NAS Linux helios64 5.8.14-rockchip64]