Here's the test I ran.

I pasted this into script editor, and started running it.

------------------
set myFolder to path to startup disk
set fileName to "Sequence 1  Copy_070530035437.mov"
set myFile to ((myFolder as string) & fileName)

try
    open for access file myFile with write permission
    set openFile to the result
on error
    close access file myFile
    open for access file myFile with write permission
    set openFile to the result
end try
set x to 1
set eof of openFile to 1
repeat
    set x to x + 1
    get eof of openFile
    write x as string to openFile
end repeat
------------------

I then pasted this into Script Debugger and started running it.

------------------
set myFolder to path to startup disk
set fileName to "Sequence 1  Copy_070530035437.mov"
set myFile to ((myFolder as string) & fileName)
get eof of file myFile

set oldSize to 0
repeat
    delay 2
    try
        set curSize to get eof of file myFile
        log curSize
        if curSize is oldSize then
            exit repeat
        end if
        set oldSize to curSize
    on error errText
        log {"error @ file size check run stat script -> user canceled",
errText}
        display dialog errText
        exit repeat
    end try
end repeat
------------------


The result was no errors, and it faithfully logged the growing file size.

get eof file "Jaguar:Sequence 1  Copy_070530035437.mov"
-->    425.0
(*425.0*)
get eof file "Jaguar:Sequence 1  Copy_070530035437.mov"
-->    467.0
(*467.0*)

When I stopped the Script Editor process it gracefully exited as expected.

Is it possible there is a return character or some other odd character in
your original file name?

>>>>Here the original(not working) file name and path:
  "/Volumes/SATA_465_01/FCP6 test folder/Sequence 1
Copy_070530035437.mov"

HTH

ES