Correct spelling, explanation: transfer is a word of Latin origin, namely the word transferre. Over time the word became anglicized, which meant losing the re suffix, as it was unnecessary, and slightly adjusting spelling and pronunciation to English standard. As of now, the double r is only found in some forms of the word’s derivation (like transferred).

Definition of transfer:
Noun
1. An act of moving a thing from one place to another (figurative and literal)
verb
1. Moving a thing from one place to another (figurative and literal)
As my room became cluttered, I decided to transfer some of my things to the attic.
Everybody liked Steve, as he seemed to transfer good energy to everyone.
2. Changing school or a workplace
Our breach didn’t bring revenue, so most employees were transferred to other divisions.

Incorrect spelling, explanation: despite etymology and other issues concerning the word transfer, the most important thing to notice is to see that the number of verbs and nouns ending in double r in English is very scarce. This is mostly caused by the fact that the other r doesn’t change the pronunciation and is simply unnecessary, which is why forms such as transferr are generally incorrect.

Incorrect spelling

Incorrect spelling

Incorrect spelling