Hello.I've been using Tixati for almost a year ,I came to Tixati from Qbittorrent.And i bought a WD MY CLOUD 4TB recently.I've had a few problems with this combination.

First problem happened when i was rebooting my computer.All files on the cloud were offline,and had to start them manually by myself in order to continue seeding.Problem got fixed somehow by itself.
Second problem was that there was no way for me to see which files are on my HDD and which ones were on my Cloud device (eventually i just had to do Select All - move all) which took awhile.
Thirst problem occurred today when i was downloading 3 files and one of them kept checking the file every second or so while downloading.This made the file take alot of time to finish downloading and i'm not even sure it's the full folder.
Also ,alot of my torrents had a different size in Bytes DL column than Bytes column.So a file that has 295M , has actually download 299M. Or a music folder of 3.64G has 9.41G downloaded.

Can someone tell me how to fix the checking file while downloading please ?

For each Transfer go to -> Details
You will see both FILE data, as well as protocol data (all the send/recv to and from peers/seeds)... More protocol data is consumed when you skip files, selectively download, or leave a download in QUEUE status (Anything not stopped/offline uses data for DHT etc)

As far as your NAS, there are reports and options in Settings to help with this. Both file copy speed and the issue of constant re-checking appear to affect many models.
If you search and try to find a repeatable process, you could help the developers by using Help -> Diagnostics page -> File log

If you only start ONE transfer, and open the log page, go through a cycle of the issue you see constantly, the developers can receive direct contacts @ support at tixati.com