1 Nephi 11 - 12

Nephi records -

"...had desired to know the things that my father had seen, and believing that the Lord was able to make them known unto me, as I sat pondering in mine heart I was caught away in the Spirit of the Lord, yea, into an exceedingly high mountain, which I never had before seen, and upon which I never had before set my foot." (Book of Mormon 1 Nephi 11:1)

Though it really doesn't matter, there are differing opinions as to whom "the Spirit of the Lord" of which Nephi refers. One opinion is that it was literally the spirit of the Savior similar to the same experience of the Brother of Jared. It seems this way as he describes that he -

"...spake unto him as a man speaketh; for I beheld that he was in the form of a man; yet nevertheless, I knew that it was the Spirit of the Lord; and he spake unto me as a man speaketh with another." (Book of Mormon 1 Nephi 11:11)

They may of supported this argument with the idea that just proceeding the vision of the birth and life of the Savior, the Spirit -

"...said unto me: Look! And I looked as if to look upon him, and I saw him not; for he had gone from before my presence." (Book of Mormon 1 Nephi 11:12)

But my personal feelings are that the Spirit of the Lord that Nephi spake to "as a man speaketh to another" was the Holy Ghost. He being called at times - the Spirit of the Lord. My feelings come from Nephi's experience that -

"...I was caught away in the Spirit of the Lord..." (Book of Mormon 1 Nephi 11:1)

Being "caught away in" the Spirit of the Lord seems to coincide with scriptures of being in the Spirit such as Lehi's comment that of -

"...being thus overcome with the Spirit" (Book of Mormon 1 Nephi 1:8)

And the great vision of Joseph Smith and Sidney Rigdon where they record also of -

"...being in the Spirit" (Doctrine and Covenants Section 76:11)

The same argument that the Spirit of the Lord said, "Look", "and he was not", to which an angel replaced him, would follow the same logic where -

"...it came to pass that I beheld that she was carried away in the Spirit; and after she had been carried away in the Spirit for the space of a time..." (Book of Mormon 1 Nephi 11:19)

As well as -

"...I beheld the heavens open, and the Holy Ghost come down out of heaven and abide upon him in the form of a dove." (Book of Mormon 1 Nephi 11:27)

The Savior himself said -

"...I tell you the truth; It is expedient for you that I go away: for if I go not away, the Comforter will not come unto you; but if I depart, I will send him unto you." (New Testament John 16:7)

This suggests to me that as Nephi witnessed the life of the Savior, it would only make sense that the Spirit of the Lord, the Comforter, or the Holy Ghost would not come or be present.

In the eternal perspective of things, of whom the Spirit of the Lord is refering to that carried Nephi away, if it was the Holy Ghost, I think that this verse would have great significance -

"...for I beheld that he was in the form of a man; yet nevertheless, I knew that it was the Spirit of the Lord.." (Book of Mormon 1 Nephi 11:11)

Which if as the Prophet taught -

"...but the Holy Ghost has not a body of flesh and bones, but is a personage of Spirit..." (Doctrine and Covenants Section 130:22)