Booting from a Specified Flash

This version of the boot command gives you the option of specifying whether

to reboot from primary or secondary flash, and is the required command for
rebooting from secondary flash. This option also executes the complete set
of subsystem self-tests.
Syntax: boot system flash < primary | secondary >
For example, to reboot the switch from secondary flash when there are no
pending configuration changes in the running-config file.