Do not select an entire volume root to be backed up. For example if you want to back up the entirety of the external volume F:, don't select F:. Instead, open up F: and select everything desired that is one level down. Do this even if you want everything.
Although the example above is from Windows, this advice applies equally to Mac and Linux.
There are several reasons for this. Doing so can confuse the application during either backup or restoration. Additionally, on some filesystems whenever any file changes, the mtime of the root volume is updated. If the backup application is watching the entire volume then this will cause it to rescan the entire volume. Depending upon the contents, the volume's mtime may well change much more frequently than the backup application can possibly rescan it, getting it stuck in a never-ending loop. So for best performance, select not the entire volume but the files and folders one level down.
Our advice to "select everything desired that is one level down" does not override our advice elsewhere to not back up your entire computer. See our article System Backup if that is what you are trying to do.
If by chance you have in error backed up an entire volume root, then likewise do not attempt to download the entire root. Doing so often fails. Instead, in the Manage tab open up the volume root and select the next-level items inside it. As you do so, keep in mind our recommendation to use download selections of 10 GB or less at a time. Also keep in mind that having backed up the volume root may have caused corruption. Corrupt files cannot be downloaded, but you can use this procedure to download any files that are not corrupt.