I'm not clear regarding your question - extract the drive for what purpose? Normally, FAS 2020/2040 NAS are setup Raid 6 with a hot spare. You can't remove a drive from a Raid environment with specifc data on it because the data is striped across multiple drives unless you're removing the hot spare that has no data on it.
Re: Can we use a SAS hard disk 450g extracted from an FAS2050 on a FAS2040?
Sorry... I only saw the "from an FAS2050" on my browser screen. As far as our experience, the SAS 450G 15K drive is the same on both models, the cage/disk slide mount is the same and should be able to function the same in both units. If NetApp has proprietary microcode on their drives, ONTap should recognize the drive in either NAS so long as the head/cyl configuration is the same as your existing drives in the FAS 2050