Whenever we migrate huge amount of emails (either between accounts or to HSM secondary storage etc), we will notice that the SDA1 drive will use over 90% of it's storage. Is this normal?

What is consuming the SDA1 actually? Temp files of these migrating emails? Will it be smart enough to limit the consumption or will it ever going consume unit it's 100%? If so, will limiting the amount of emails moved at one goes help to reduce this?