A unique sandbox folder is created each time a custom update is installed and for updates that require special processing. The sandbox folder is located on the target machine here:
Contained within a sandbox folder will be several files you might find useful for debugging purposes. This includes all the files that were published in the CAB file as well as any files you specified on the Custom Files tab.
The sandbox folders are only temporary and will be automatically removed after a number of days. For this reason, if there is a sandbox folder you would like to preserve you should copy it to a different location.
Finally, custom updates are not the only updates that use sandbox folders. Sandbox folders are also created for updates that require special handling so be careful to select the correct sandbox folder during your debugging process.