Yes, swapping the Xbox 360 Hard Drive loses your Black Ops multiplayer data (rank, unlocks, stats, leaderboards) stored on that drive.
Black Ops stats are also stored server-side (Demonware/Treyarch). Rank/Prestige/Leaderboards *should* sync on first login if profile is same. But Custom Classes, Camo Progress, Combat Record details, Theater films are local-only. Backup local save.