The Doctor and Donna arrive at the Library, an entire planet that is a library. The Doctor notes that it's odd that there are no other people in the library. He checks a terminal and finds out that there are trillions of other life forms. A team of rescuers arrive to help and the group finds out that those life forms are Vashta Nerada, a life form that appears as shadows and devours humans.
The computer system that runs the Library, CAL, noticed that the patrons were in danger, so she saved all 4,022 humans who were in the library. She has more memory space to transfer them to another location. The Doctor offers to let CAL use his mind as extra memory space. River Song, one of the rescuers, refuses to let The Doctor sacrifice his mind and lets CAL use hers.
CAL successfully transfers everyone back to the Library, although River dies in the process.
Ensuring you have enough storage space is essential for managing your data. Storage devices need room to process transfers and to run updates and other maintenance operations. Storage devices also break down over time, so it's important to migrate files to new devices after a few years.
Read more on the Storage and Back Up page on the Data Management Best Practices guide.