Not able to backup or restore.

On my last backup of my HP CQ5115uk computer
a caption [Windows Backup skipped F:\because it
can not be found on drive F:\.] (Windows 7 64bit).
A further two backups has resulted in my being
unable to further backup or restore my computer.
F drive is an external 1Tb USB drive, and this drive
is not connected to the computer other than when
required.
Can anyone please, help me in this error?

 

Many thanks for your assistance.
The first backup, the F external drive was
connected to the USB socket, other backups
since then, F external drive was disconnected
from the USB socket.
I have Internal drive lettered C, D.
External drives USBs F, G, I.
External drive I is used for the Backups and is
permanently connected to a USB socket.
External drives F and G only connected USB socket
as when required i.e. to save or download files/folders.

The F external drive has mostly software titles and files
stored.