Well, I don't really know exactly what's going on either, but I do know for sure that the server *is* returning a 550 error, and the file *is* getting uploaded. Here's my log from SmartFTP where I manually uploaded the file (on both an IIS 6 FTP server and a Linux ProFTP server):
IIS 6 FTP server
==================================
[21:42:25] SmartFTP v2.0.1001.0
[21:42:26] Resolving host name "blah.net"
[21:42:26] Connecting to xx.xx.xx.xx Port: 21
[21:42:26] Connected to balh.net.
[21:42:26] 220 Microsoft FTP Service
[21:42:26] USER xxxxxxx
[21:42:26] 331 Password required for xxxxxxx.
[21:42:26] PASS (hidden)
[21:42:26] 230 User xxxxxxx logged in.
[21:42:26] SYST
[21:42:26] 215 Windows_NT
[21:42:26] Detected Server Type: Windows NT
[21:42:26] FEAT
[21:42:26] 211-FEAT
[21:42:26] SIZE
[21:42:26] MDTM
[21:42:26] 211 END
[21:42:26] TYPE I
[21:42:26] 200 Type set to I.
[21:42:26] REST 0
[21:42:26] 350 Restarting at 0.
[21:42:26] PWD
[21:42:26] 257 "/blah" is current directory.
[21:42:26] CWD /blah/Users/blah/Backup
[21:42:26] 250 CWD command successful.
[21:42:26] PWD
[21:42:26] 257 "/blah/Users/blah/Backup" is current directory.
[21:42:26] TYPE A
[21:42:26] 200 Type set to A.
[21:42:26] PASV
[21:42:26] 227 Entering Passive Mode (192,168,1,7,16,99).
[21:42:27] Opening data connection to 192.168.1.7 Port: 4195
[21:42:27] LIST -aL
[21:42:39] 425 Can't open data connection.
[21:42:39] Automatic failover of data connection mode from "Passive Mode (PASV)" to "Active Mode (PORT)".
[21:42:39] PORT 192,168,1,151,5,69
[21:42:39] 200 PORT command successful.
[21:42:39] LIST -aL
[21:42:39] 150 Opening ASCII mode data connection for /bin/ls.
[21:42:39] 0 bytes transferred. (N/A/s) (0 ms)
[21:42:39] 226 Transfer complete.
[21:43:29] NOOP
[21:43:29] 200 NOOP command successful.
[21:43:36] Remote file exist check: "BackBurner.200703012129.zip".
[21:43:36] TYPE I
[21:43:36] 200 Type set to I.
[21:43:36] SIZE BackBurner.200703012129.zip
[21:43:36] 550 BackBurner.200703012129.zip: The system cannot find the file specified.
[21:43:36] File size check (SIZE) failed. File not found or permission denied.
[21:43:36] PORT 192,168,1,151,5,71
[21:43:36] 200 PORT command successful.
[21:43:36] STOR BackBurner.200703012129.zip
[21:43:36] 150 Opening BINARY mode data connection for BackBurner.200703012129.zip.
[21:43:38] 3030756 bytes transferred. (1.30 MB/s) (00:00:02)
[21:43:38] 226 Transfer complete.
[21:43:38] MDTM 20070302023005 BackBurner.200703012129.zip
[21:43:38] 550 20070302023005 BackBurner.200703012129.zip: The system cannot find the file specified.
[21:43:38] SIZE BackBurner.200703012129.zip
[21:43:39] 213 3030756
[21:43:39] TYPE A
[21:43:39] 200 Type set to A.
[21:43:39] PORT 192,168,1,151,5,73
[21:43:39] 200 PORT command successful.
[21:43:39] LIST -aL
[21:43:39] 150 Opening ASCII mode data connection for /bin/ls.
[21:43:39] 68 bytes transferred. (N/A/s) (0 ms)
[21:43:39] 226 Transfer complete.
[21:43:39] Transfer successful.
Linux ProFTP Server
=====================================
[21:53:49] Resolving host name "blah.com"
[21:53:49] Connecting to xx.xx.xx.xx Port: 21
[21:53:49] Connected to blah.com.
[21:53:49] 220 ProFTPD 1.2.10 Server (ProFTPD) [xx.xx.xx.xx]
[21:53:49] USER xxxxxxxx
[21:53:49] 331 Password required for xxxxxxxx.
[21:53:49] PASS (hidden)
[21:53:50] 230 User xxxxxxxx logged in.
[21:53:50] SYST
[21:53:50] 215 UNIX Type: L8
[21:53:50] Detected Server Type: UNIX
[21:53:50] FEAT
[21:53:50] 211-Features:
[21:53:50] MDTM
[21:53:50] REST STREAM
[21:53:50] SIZE
[21:53:50] 211 End
[21:53:50] PWD
[21:53:50] 257 "/" is current directory.
[21:54:12] CWD /httpdocs/media/temp
[21:54:12] 250 CWD command successful
[21:54:12] PWD
[21:54:12] 257 "/httpdocs/media/temp" is current directory.
[21:54:12] Remote file exist check: "BackBurner.200703012129.zip".
[21:54:12] TYPE I
[21:54:12] 200 Type set to I
[21:54:12] SIZE BackBurner.200703012129.zip
[21:54:12] 550 BackBurner.200703012129.zip: No such file or directory
[21:54:12] File size check (SIZE) failed. File not found or permission denied.
[21:54:12] PASV
[21:54:12] 227 Entering Passive Mode (82,165,240,21,200,29).
[21:54:12] Opening data connection to 82.165.240.21 Port: 51229
[21:54:12] STOR BackBurner.200703012129.zip
[21:54:12] 150 Opening BINARY mode data connection for BackBurner.200703012129.zip
[21:55:22] 3030756 bytes transferred. (42.2 KB/s) (00:01:09)
[21:55:23] 226 Transfer complete.
[21:55:23] MDTM 20070302023005 BackBurner.200703012129.zip
[21:55:23] 550 20070302023005 BackBurner.200703012129.zip: No such file or directory
[21:55:23] SIZE BackBurner.200703012129.zip
[21:55:23] 213 3030756
[21:55:23] TYPE A
[21:55:23] 200 Type set to A
[21:55:23] PASV
[21:55:23] 227 Entering Passive Mode (82,165,240,21,200,45).
[21:55:23] Opening data connection to 82.165.240.21 Port: 51245
[21:55:23] LIST -aL
[21:55:23] 150 Opening ASCII mode data connection for file list
[21:55:23] 210 bytes transferred. (3.25 KB/s) (63 ms)
[21:55:23] 226-Transfer complete.
[21:55:23] 226 Quotas off
[21:55:23] Transfer successful.
I highlighted the pertinent lines from the log above. It seems to me like its somehow just ignoring the 550 errors that are returned. Is there some way I can do something similar with the FTPConnection.UploadFile method? I really need to figure out how to get past this and get this upload working correctly. I'm pretty much at a standstill at the moment. Any ideas would be greatly appreciated.