We have already discussed how to intentionally do a "partial" download. But maybe it's NOT on purpose. You may have sent a large number of rolls to the lab, and a full download will run into the gigabytes. Sometimes, during such lengthy sessions, Bad Things Happen. You can have a power failure, loss of internet, the lab server might go down--there are plenty of possibilities.
The time already spent has not been wasted. If you leave the partial download where it is in the destination folder, FTProofsheet can pick up where you left off.
When the issue is corrected, restart the download. When FTProofsheet detects that certain files already exist in the destination, it will check that they are complete. If so, FTProofsheet will just move on to the next. If not, the download of that file will be picked up from the last byte on the destination. When past all that, the download will continue normally