btmp logs failed attempts into your system. In theory this could mean someone is trying to brute force passwords. You can empty this file but should not remove it since software could crash when it expects the file to be there. To empty it from command line:
sudo > /var/log/btmp
If it gets filled quickly you really should investigate why.
driveclient.log is something specific to space rack. Again to empty it use...
sudo > /var/log/driveclient.log
Regarding this last you should have the following files (nore info in the link):
Agent file location (Linux)
Assuming a default installation, following are the agent file
locations on Linux systems:
Configuration files: /etc/driveclient
Startup script: /etc/init.d
PID file for running the agent: /var/run/driveclient.pid
Database: search for a *.db file under /var/cache/driveclient
You can turn logging off or set the size of this log in /etc/driveclient and also how many compressed backups will be saved before the next one gets deleted.