@mastermind,
Regarding derivatives, your observation is right that MTv3 backfills data from very first trade in the particular contract being backfilled. However, it does not deletes/over-rides any historical data from Amibroker database (except previous trading session's data). We have not observed such problem before, so please take remote support by sharing your computer over internet remote access.
Also the machine should not become unresponsive, because both Amibroker and MTv3 are extremely resource efficient applications. It's hard to comment before diagnosing your system through remote access.
Regarding derivatives, your observation is right that MTv3 backfills data from very first trade in the particular contract being backfilled. However, it does not deletes/over-rides any historical data from Amibroker database (except previous trading session's data). We have not observed such problem before, so please take remote support by sharing your computer over internet remote access.
Also the machine should not become unresponsive, because both Amibroker and MTv3 are extremely resource efficient applications. It's hard to comment before diagnosing your system through remote access.