We have succesfully defragged dynamic disk with no problems. 250GB was the max. 725 will probably take the weekend to finish.
Analyze the drive first, look at the report and see if you can clean up some junk files and then defrag.
The best thing you can do is to backup all the important data or files stored on the hard disk drive before performing the defragmentation procedure. However, you can still continue defragging the files even if you cannot create a backup on them because defragmenting the hard disk drive doesn’t affect the files stored on it.