Tool ? It depends on the type of data you are migrating.
In case it's SAN we have DTA ( Migration appliance which migrates the data - You will have both the options offline/online).
In case it's NAS then you can use securecopy which is used by NetApp PS for migration. You can also use other native migration tools like Robocopy/rsync etc. to do the same.
In case you don't wish to use the DTA appliance you can do the SAN migration at the host level using native tools via mirroring etc.
I hope it helps !