What is the most likely reason a systems administrator cannot back up user files?

The most likely reason a systems administrator cannot back up user files is that the files are currently open. When files are open, especially in applications that maintain a lock on the file (such as databases or documents being edited), the operating system may prevent any processes, including backup operations, from accessing those files to ensure data integrity. Backing up open files can lead to inconsistent or corrupt backup copies, as the files may change during the backup process.

In contrast, files that have been copied, modified, or compressed generally do not prevent a backup from occurring. A copied file is simply a duplicate and can be backed up like any other file. Similarly, modified files can still be backed up, and the backup may capture the most recent version of the file. Compressed files, while handled differently by backup software, are also not typically a barrier to performing a backup. Thus, the state of a file being open presents a unique situation that can hinder the backup process.