Q:

lsblk and df -H don't match

Office machine - After running lsblk I noticed I am not using all the disk space that is one the machine,
NAME   MAJ:MIN RM   SIZE RO TYPE MOUNTPOINT
sdb      8:16   0 465.8G  0 disk 
├─sdb2   8:18   0     1K  0 part 
├─sdb5   8:21   0  63.9G  0 part [SWAP]
└─sdb1   8:17   0 401.9G  0 part /
loop6    7:6    0  86.6M  1 loop /snap/core/4486
loop4    7:4    0 101.9M  1 loop /snap/skype/23
loop2    7:2    0  86.6M  1 loop /snap/core/4571
loop0    7:0    0   137M  1 loop /snap/skype/33
sda      8:0    0   2.7T  0 disk 
loop5    7:5    0  86.6M  1 loop /snap/core/4650
loop3    7:3    0 146.2M  1 loop /snap/skype/30

And when I call df -H,
Filesystem      Size  Used Avail Use% Mounted on
udev             34G     0   34G   0% /dev
tmpfs           6.8G  652M  6.1G  10% /run
/dev/sdb1       425G   76G  328G  19% /
tmpfs            34G   39M   34G   1% /dev/shm
tmpfs           5.3M  4.1k  5.3M   1% /run/lock
tmpfs            34G     0   34G   0% /sys/fs/cgroup
/dev/loop4      107M  107M     0 100% /snap/skype/23
tmpfs           6.8G  173k  6.8G   1% /run/user/1001
/dev/loop6       91M   91M     0 100% /snap/core/4486
/dev/loop3      154M  154M     0 100% /snap/skype/30
/dev/loop2       91M   91M     0 100% /snap/core/4571
/dev/loop5       91M   91M     0 100% /snap/core/4650
/dev/loop0      144M  144M     0 100% /snap/skype/33

I wonder why the sda      8:0    0   2.7T  0 disk is not showing up here.  How can I safely add (mount) or use this space?  And why it is not showing up in df -H? 

A:

Because sda      8:0    0   2.7T  0 disk doesn't have a mountpoint.  df handles only mounted filesystems.  So if it's unmounted it won't show up.   For that same reason, the /dev/sdb2 doesn't show up also. 
Also  /dev/sda seems to have no partition table there.  If you think it should, check the disk health.  Otherwise, if it doesn't have a partition table, create one.  Mounting can be done via file manager or udisksctl mount -b /dev/sda1 for partition sda1 for example

