Chop off end of file

Sat Jun 25 23:57:36 GMT 2005

I have a ~70GB, of which the last ~50GB is garbage. I know exactly at what 
byte I want to cut the file, but I do not have enough free diskspace to do 
a 

   dd if=orig of=pre bs=... count=....

Is it possible to just chop off the end of a file, without making a copy 
of its beginning? It's a dv-file, resulting from "fwcontrol -R". I left my 
home and forgot about it. When I returned home, the disk was full, and now 
I need to delete the garbage. I know at exactly what byte to cut the 
stream.

Svein Halvor