It's possible that your LBA has changed from 32bit (MBR) or 64bit (GPT), to 28bit - although you would actually see slightly less than 139GB.
When you use the world "reflect" I assume that you've adopted it from the 3rd party software that you used? Many people love those things but I'm convinced that many of them are somehow invested saying that they do. I certainly don't. If you do a quick search on here or google, you'll find that most of the partition table faults that people suffer with their hard disks come down to using the wrong 3rd party software for what they are trying to achieve, or using perfectly good software in the wrong way for what they are trying to achieve.
You haven't given a great amount of detail here about either of your drives, the software that you used, how you used it or exactly what you were trying to achieve. It seems as if you were trying to take your C: and mirror the entire drive, filesystem and all, onto your extenal drive. That could be for the purposes of backng up or perhaps you are looking to swap out your C: without reinstalling windows.
Either way, ( and to the best of my knowledge) you need to have a hard drive of equal or greater capacity to mirror onto, and with the same sized block sectors (the vast majority of contemtorary disks use 512byte logical blocks, shouldn't be an issue with a 600GB drive but some WD drives over 2.12TB use 4096byte logical blocks).
If you're attempting to mirror an ancient drive though, or something quite exotic, then you may be mirroring a partition table that is not suited to your external drive. Old ATA and IDE drives may use these kinds of LBAs. If that is the case then you don't want to be mirroring the drive, you want the contents and not the filesystem.
If not, it's software or user error (most likely). Regardless of which, you don't need to worry about data loss so delete your dodgy partition table and start again...