Complex sentence: After the first recording session on 6 June 1962, Martin had one reservation, as he felt the recording would be better served by using an experienced studio session drummer (which was normal practice at the time), instead of Best.
Split into simpler sentences: After the first recording session on 6 June 1962, Martin had one reservation. Typically for the time, he felt the recording would be better served by using an experienced studio session drummer instead of Best.