The master, model, and sybsystemprocs databases do not have separate segments for their transaction logs. Use dump transaction with truncate_only to purge the log, then use dump database to back up the database.
Backups of the master database are needed for recovery procedures in case of a failure that affects the master database. See the System Administration Guide for step-by-step instructions for backing up and restoring the master database.
If you are using removable media for backups, the entire master database must fit on a single volume, unless you have another SAP ASE server that can respond to volume change messages.