ShadowProtect backups fail with an error "platform error writing the file (-121 The semaphore timeout period has expired.)"
The specific error: (-121 The semaphore timeout period has expired.), is an indication that there is a problem connecting to the storage location. Most times being a network location but can be other destinations. Note that you may receive semaphore timeouts when reading from the Shadow Copy Storage area on a local machine as well, as those locations are reached via internal UNC path to the \\STC_SNAPSHOT_VOLUME address area.
If you are receiving the error on WRITE to a destination, the first recommended step would be to try targeting a different destination to ensure the problem does not exist there. For all other issues, please reference the following Microsoft KB article: http://support.microsoft.com/default.aspx/kb/325487