Some software (you didn't tell what you installed) can do that. Now go into disk management (part of Windows) and fix it by deleting all partitions and making a new one. That's the easy part.
Restoring existing files is the difficult part, and you won't get 100% back. That's a well-known thing of external disks: never use them for data storage, only as one of your backup devices and to move data between systems. If you follow that simple rule, you'll never lose anything.
Now head to the http://forums.cnet.com/storage-forum/ and see if any of the (free, free-to-try or paid) software in the top 2 posts can extract some of your files to the internal hard disk of your PC (or to another new external disk).
And if you want to use the recovery part of that software you downloaded, better buy a 32 GB USB-stick and use that.
Kees
The problem is I installing a software from a different USB. It's pop-up a window and ask something about recovery and I click yes. It automatically detect my external HDD as a place to recovery and format the HDD. After realizing that I click cancel and notice that my external HDD from 1Tb became 32Gb. Anyone know how can I get my external HDD back to 1TB and if possible can I restore all the files that are lost

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