My guess is that there is a particular 'problem' file in your pictures files.  Several cases have been reported with such a 'freeze' always at the same % of the size calculating.

Offending files are often particular video ones, or files with incorrect characters or file extensions (several dots, slashes ...)

Since you have already repaired and optimized, you could try the following:

- locate your catalog on your computer : in the help menu, option system info, you get its location.

- make a copy of the catalog (database and accessory files) : that's the whole folder containing the database 'catalog.pse10db'. Name the new folder something like 'My catal copy'. You can use it just like the original one by double clicking on the catalog.pse10db file.

- using this catalog, select ONLY the video files in the view menu, and delete them from the catalog, NOT from the drive, obviously.

- try the backup without the video files.

Depending on your results, we'll see what we can do next.

As I said, there must be 'a particular problem file' in your video pictures. As a matter of fact, probably an unrecognized format, such as a video from a particular camera or cell phone.

So the idea to find the culprit(s) is to do another copy of your original catalog folder, and this time, select for the backup all the video files except one file format like .3gp, .mv4 .

You'll have to repeat with other formats until you find the format type which is blocking the calculation.

Another guess : I have seen that in PSE10 the backup of files is made in chronological order of the pictures. Not absolutely sure about that, but the renamed B0000001.jpg etc in the backup folder are in chronological order. That would mean that the offending file or file format blocking at 88% is a rather recent one.

With PSE6, the files in the backup folder were in alphabetical order...