Access is denied because the NTFS volume is already exclusively opened

Solution 1:

it means in general that your device has been already mounted (automatically, for example, when detected).Depending on what displays 'mount' command, you can 'umount' your device, and then try to remount it as you want.

Solution 2:

I'm trying to mount a drive with:

sudo mount -t ntfs /dev/sda1 usb

That is not the drive you think it is. Look:

Update: the command mount responds with:

[...]
/dev/sda1 on / type ext4 (rw,relatime,errors=remount-ro,data=ordered)
[...]

/dev/sda1 is already mounted and it is your root drive which contains the main file system of your Ubuntu installation. It is not a NTFS partition of an external USB drive (I assume you want to mount a USB drive because you created a mount point called usb).

You shall check the partition tables of all drives with:

sudo fdisk -l

If you are using GPT for any of the drives, you might need to run:

sudo parted -l

Probably the partition you want to access is /dev/sdb1.