SyncToy. Free tool from MS. I use it to backup to my external hdd (4 TB). But it can be also used to copy over to the NAS and in the future to backup the nas.
This tool, which I just downloaded and installed, looks pretty good. Nice interface, evidently intuitive, easy to use, very simple to install.
I've been having problems copying data from the problem drive to another USB HD, which I want to do before moving data to the NAS. The problems involve Windows Copy command balking repeatedly, and this mystifies me because I copied that data over to the HD in the first place with no problems using Windows Copy command. I get errors mostly concerning excessively long file/folder names. I fix the problem, start all over, another such problem happens, it's a bitch.
After I finally get all the data to the other USB HD, I plan to run Synology's own Storage Manager disk checking, both Quick and Extended to gain some assurance that it's HDs are OK, then I will copy data anew and have fingers crossed that these problems are behind me.
I have SyncToy running right now, copying the folder (with many many subfolders) that's been giving me a lot of problems, doing an "Echo" to a folder on the other USB HD, the folder that wasn't getting everything using Copy. Presumably it will not choke on the same problems.
I figure I'll either use SyncToy to copy data from the NAS to USB HDs to be stored off site or possibly Synology's own apps to do that, if they are nice and functional, I suppose that exists. But SyncToy may be all I need for that. A Synology support guy the other day told me to use Windows Explorer to copy/move/etc. files, folders, whatever, not Synology's File Station. He didn't elaborate, he obviously was pretty burned out on explaining things to people, he preferred to just tell you what to do. I get the feeling that Synology may not be the swingingest, happiest place to work. I used to do tech support, did so twice. The first time the people I dealt with were by and large very technically astute people, and it was software, the second time they were employees of the company I worked for and I was the expert, again software (and I knew the code, had written a lot of it). Both were good experiences. But doing tech support for customers of a hardware+softwares product like Synology's NAS's, that must be very complex.