On The Fly
The term, "On The Fly," in PC terms, is when a computer computes results in real time.
On The Fly is usually referred to when making copies of removable media (DVDs, CD, Blu-rays) without saving anything to hard drives first.
An example would be copying a DVD that is in one drive, while writing the same copied media to a second disc in a second drive.
The same term is used when encrypting data storage, when the data is encrypted while it is being written. For that, the term OTFE (on the fly encryption) is used.
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Related software tools
Nero Burning ROM (Shareware)
Nero is the best and most complete DVDR / CDR / Blu-ray all-in-one software
Nero BurnLite (Freeware)
Nero Free Edition - Enjoy basic data burning and copying capabilities for your CDs and DVDs, using this freeware version of Nero.