Depends on a number of things...
- Amount of data you need to copy ?
- What other systems you have available and the amount of free space on those systems?
- Do you have an external hard drive you could connect to one of your other systems ?
This is how I did it, might work for you might not, depends on your hardware....
- Connect an external hard drive up to a computer on the network, in my case it was a 4TB USB connected to a Windows 7 Laptop (I will call this "slave").
- Use ROBOCOPY to copy the files from your server to the external hard drive which is connected to "slave". If you are using a *NIX/FreeBSD based system instead of Windows use rsync
- Take the hard drives out of your server, add them to the FreeNAS box and configure as required
- Make sure the volume on the FreeNAS box is available (shared) either NFS or CIFS.
- Connect "slave" system to share on FreeNAS and use ROBOCOPY (or rsync if using *NIX/FreeBSD) to migrate the saved data to FreeNAS.
- Keep the data on the external hard drive as an archive. External H/D's are relatively cheap nowadays. All depends on how much you value your data of course.
ROBOCOPY has a few nice functions whereby you can mirror (/mir) a complete directory structure, plus, if you have to stop it for some reason you can re-run it and it will figure out what has already been copied and ignore, thus starting from where it left off. Other nice features are /W and /R (wait and retry). Some files can be locked and can't be copied, /W says "wait for x number of seconds", /R means "retry X number of times". When both timeout, the backup just moves to the next file.