My guess is that he joined the 7th Cav. without being discharged from his previous unit. The 7th was notorious for accepting these "self-transfers". I'd guess that is what the court martial was about. As this was about the time that the 7th took in a huge number of illegal transfers, and his status was probably discovered. He may have even gone AWOL when discovered, to avoid the consequences.

Some of the illegal transfers were later OK'ed by Forrest and the men were allowed to stay in the 7th, but I think all of those were transfers from other cavalry units that had come under Forrest's command. It would not have been possible for him to have authorized illegal transfers from the infantry (nor, could he have authorized the transfers from cavalry units that were not then under his command).

That not withstanding, I further surmise, that later the soldier/trooper was attached to Hardies Reserves, but was still assigned to the 7th. I find a lot of those situations for men of the 7th in April 1865. At least some were home to recuperate, and attached to the reserves while home. These men usually signed their paroles as members of the 7th Cav., even though they were operating as a member of a reserve unit.

Here is an example of a man that "self-transferred" to the 7th, and was marked on the muster roll of his proper unit as "AWOL, serving with the 7th Ala. Cav.":

LOCK, Thomas Vanduran [or Van Buren], Private (Born 5/12/1837. Mustered in Vienna on May 19, 1861 in the 2nd Alabama Infantry Regt. May have transferred to 2nd Ala. Cav. Enlisted in Dupree's Independent Cavalry Company [aka Company D, 12th Miss. and Ala. Cav.] at Pickensville 9/1/1863. Noted in Mt. Zion Church records as killed in action in 1863. Listed on muster roll as AWOL, and serving with the 7th Ala. Cav. without permission 6/5/1864. There is a grave marker in the Bunn Cemetery in Bethany that states he died in 1914 and another source lists his date of death as 5/30/1915. It appears that the death report of 1863 was just incorrect.)

A very large number of men from Dupree's Company "self-transferred" to the 7th Cav. Regt. Forrest later made some of the transfers official. Much has been made of the fact that after Gen. Hood’s retreat from Nashville to Corinth, only 64 rank and file were left of the 350 with which the 7th began its service. However, I think that it was during this time frame that the 7th was discovered to have accepted so many illegal transferees. These men being sent back to their proper units may have constituted a large part of the reduction in numbers in its ranks. After recruiting a few weeks, the regiment joined Gen. Buford, at Montevallo with 300 men.