You are about to recompress the photographs using a different scheme. Depending upon the number of files and the types of compression, this could take a significant amount of time and cannot be interrupted. Before proceeding with any major activity such as this, it is always wise to back-up your hard disk to protect against power failures and other hardware and software problems.
The program will make a check that you have enough disk space to complete the operation. For more information click on recompression.