Unable to mount an external USB HD

Friends, I have an external USB HD (740 GB). The disc stopped working in Windows. So I decided to recover the data using Knoppix 5.1 Live CD. Knoppix recognized the disk, and I was able to recover most of the data. Because of the lack of time, it took me 6 months to copy the data from the disk. Today I wanted to copy a couple of last folders, alas after copying 60 MBs, the disk stalked. I re-connected the disk, but it has never appeared again. I tried to reboot Knoppix, but without any success. I cannot see the disk. Please advice what should I do. Is it possible to format the disk? Please help. Thank you very much in advance.

Greetings nonameee

Warning! I am not an expert but I have encountered such problems "resolved not by my poor knowledge".

You say
"I tried to REBOOT Knoppix, but without any success"
Do you mean that your "USB HDD" was connected while booting?
If so
1°) Disconnect it and swich off your computer
2)) Wait a few minutes
3°) Try to boot your live "5.1"
This may solve your "live_CD_DVD_booting"
4°) After complete booting,connect your Usb Hdd
5°) Wether or not your USB HDD is recognized run WITHOUT DOOING ANYTHING
a) QParted or
b) GParted or GnomeParted
Just to see if your Usb is appearing (not mounted generally in my cases).

6°) If it appears all chances exist to get it again by some way to find
a) Then try to connect it with the Wndows_OS
b) If recognized by Windows
b1) Run twice the CHKDSK /f (you may test without /f before the twice ones)
b2) Run Defrag properly (you MUST HAVE TIME!...)
b3) Get your data
b4) Switch Off your usb_Hdd by the "Windows_usb_Safety_mode"
(Very IMPORTANT...in my situations)
c) If not recognized by Windows you must be very PATIENT to look for OTHER WAYS
Why not try then with mac_OS or another OS's? Such as:
TrueImage (a backup program) or
Norton Ghost etc...

You may test numerous programs.
FORMATING with precipitation your Usb Hdd is the very last thing to do.
Only in the DESPERATE CASE

I do not appear regularly on the forum by lack of time...LIKE YOU!
But I shall follow the evolution of your problem

Greetings
knp
.