I backup my PC using Nero Backitup, It took 5 hours for full computer backup, I calculated the storage size I’ve used for the hard disk and its = C:/62 GB D:/91.2 GB = C+D= 153.2 GB and when the backup ended I saw my external Hard Disk “backup” equals 138 GB, so where is the 15.2 GB !!!!!!! Did it compressed or what ???



It must have been compressed. You can restore the backup to an external drive to test and verify it.
Also note that some backup software do not backup System Volume Information and Recycle Bin folders.
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