[AnotherCat]

I have had no problems with other Samsung Tab As and I doubt whether there is any issue with S6s in general as I know others use them with Calibre. So it is likely either a fault in your particular S6 or you are suffering from the issue that exists between some individual devices (individual devices not a particular model, e.g. so not all S6s because I know others don't have the same problem with them) and Calibre for MTP file transfers. There has been a string of threads about this problem over the years.

Usually the issue shows up by Calibre insisting on doing a full rescan of the whole device's storage for books every time the device is connected (which can take quite some minutes if one has a lot of music and images in storage), rather than scan just the folders that one points it too in the device configuration in Calibre (with the device connected, in the drop down menu for Device in Calibre's toolbar), but sometimes by unreliable book transfers.

This happens even though all other file transfers and other media management applications all work correctly.

I can only suggest that you check that you have Calibre configured to only scan the folder(s) on the device containing books and that Calibre completes a scan of only those. Other than that, if the above issue is the problem, then you are on your own because I nor anyone else (as far as I am aware) has solved the problem.

I encountered the issue with a phone, all other devices worked fine with Calibre. A few times I thought I may have beaten it but that didn't last past a restart - I even factory rest the phone and spent hours setting it up again. It ended up that I got around the problem in a drastic way by falling off a boat into the salty sea 6 months ago drowning that phone. I have bought myself 2 phones since then and both work flawlessly with Calibre as do our Samsung tablets.

Basically, if you have USB on the tablet set to file transfer, as you say you do, and other file transfers and reads work (especially if they are media management or player related between the tablet and the PC) then I think it is unlikely that the issue is with your tablet.