After installing WSUS on SBS 2003 I was unable to get any updates to download. The application event log showed the following error:
Event Type: Error
Event Source: Windows Server Update Services
Event Category: Synchronization
Event ID: 364
Description:
Content file download failed. Reason: Access is denied. Source File: /msdownload/update/v3-19990518/cabpool/windowsinstaller-kb893803-v2-x86_830994754ba721add8a13bd0266d2e092f21cab0.exe Destination File: d:\WSUS\WsusContent\B0\830994754BA721ADD8A13BD0266D2E092F21CAB0.exe.
After checking the Network Service account had the appropriate permission on the destination folder, it was time to Google. Turns out that the Network Service account also needs at least Read permission on the root of the drive the content is stored on.
Thanks to the WSUS Wiki for helping me out on this one.