Sqlserver 2000, NT4

I want to growth a database's size from 28 Go to 30 Go.
("alter database" command or enterprise manager interface)

After this change, I observe a size of 30 Go in system file (.mdf), but enterprise manager signify a size of 28 Go, as well as direct query in system tables (sysaltfiles), as well as "sp-spaceused" command.

A calculation in this database return an error signifiying PRIMARY group file is full.

Help !!