Why does SpiderOak need so much disk space to back up a file?

In order to back up any file, SpiderOak must make an encrypted copy of it which is temporarily held on your hard disk. For example, imagine you have a 10 GB file that you have selected for backup. SpiderOak will make a temporary copy of it, so you will need at least 10 GB of free disk space to accommodate this. SpiderOak will then encrypt the copy and upload it. The temporary copy is retained until our servers notify the program that the transaction has completed. After that, the temporary file will be removed, freeing the 10 GB of disk space that had been used.