All is working but for @ 400 GB it's saying about a day?
Transfer is 6.7 mg secound .
Local connection at100 Mbps
Am a novice so wondering if this is ok or I should be doing it a different way?
If you use 100 Mbps connection 6-7 MB/s is more or less normal.
If possible you should use a GBit connection. When I once did set up a GBit ethernet connection I could copy files to DNS-327L with about 65 MB/sec so transferring your 400 GB should only take around 105 minutes...
But to do this, you need a GBit compatible Router and PC
If you do not have a GBit compatible Router but a PC you can also use a direct ethernet connection between NAS and PC. This is also shortly described in the manual. Usually you need a cross-over ethernet cable but modern PC LAN chipsets also support this via regular un-crossed ethernet cables.