Sometimes an FTP session is broken before you can completely retrieve a file from the MSS. If your client supports it, you can restart a file retrieval at a specific byte offset and just fetch the missing portion. The commands to do this are FTP Client specific, but one client uses this sequence:

ftp> restart 50000
restarting at 50000.
ftp> get ftptest2
local: ftptest2 remote: ftptest2
200 PORT command ok.
350-Restarting at 50000.
350 Send RETR to initiate transfer.
125 Data connection open; transfer starting.
226 Binary Transfer complete.
309984 bytes received in 0.00193 secs (1.6e+05 Kbytes/sec)

Some FTP Clients may be able to do this automatically for you. You may also be able to simulate this manually by writing the missing portion to a second file then manually joining the pieces together. For example:

ftp> quote rest 75000
350-Restarting at 75000.
350 Send RETR to initiate transfer.
ftp> get ftptest2 localfile.part2