We have documented a bug in certain builds of the GDC TMS where the TMS will suddenly start responding with The source package wasn't found when attempting a content transfer. 

The cause of this problem appears to be that the TMS is randomly changing the data directory from its actual location (ie. the storage drive) to the root drive of the TMS.  In the illustration below, the actual content is stored on E:\data\repository, yet the TMS has changed it to below:

The problem is corrected by changing the Data Directory to the root address of the drive (or RAID) where the content is actually stored:

It is important that you list the root drive, not the subdirectory where the content is stored.

After making this change, rebooting the TMS program is required.