The most commonly encountered problems during upload (and how to address them) are:
- The PHP
upload_max_filesizeconfiguration parameter is set too low
- On a lot of hosting this defaults to 2M which is generally too low these days. Either refer to your host support for details on how to increase this to a more reasonable value (8M is a good choice) or use the second FTP based upload method.
- Not enough disk space
- This can happen for any of the upload methods but will be more or less obvious depending on the method used. When using the preferred method, WordPress will usually indicate a problem with unzipping or with copying a file and will back out gracefully. When using the FTP upload it may just stop as if complete without indicating any problem which is why you need to check the FTP client upload logs. In either case, you need to make more space available by deleting any unwanted stuff or by increasing your disk quota.
Note: You need to have enough disk space for your backups. Unless you only intend to make Database Only backups, you should take this opportunity to make sufficient space available to be able to make the backups you require. You need at least as much space as the size of your WordPress site to be able to make a Full backup along with some headroom. Generally, you will want to give yourself much more than that to allow for multiple backups and an expanding site.