https://support.symantec.com/en_US/article.TECH225180.html

Cause

The 'format' utility has a new feature beginning in Solaris 10 6/06 Operating System called 'in use checking'.

This feature is enabled by default.
Solution

Any time the 'format' utility is run on the disk, it performs many verification/checks on the disk itself (does it contain a slice that is mounted, is it part of SVM metadevice, is it being used by ZFS filesystem, is it being used by swap etc...)

So, if there are lots of devices, or if there were some kind of issue on the device itself (for example, differences on what the label says and what the actual device stated), 'format' command may appears hang as it does all sort of checking in the background.

So, to avoid this kind of situation, there is an environment variable 'NOINUSE_CHECK' which can be set / turned on, prior of running the 'format' command:

# NOINUSE_CHECK=1
# export NOINUSE_CHECK
# format

Once that is set, the 'format' command will proceed with no time by passing all of the verifications and checking.