Disconnections when using the wireiess device are usually caused by poor wifi connections. I suggest that you do a WiFi channel usage survey to see whether your router is using a very busy channel and if some less busy channels are available. I use "WiFiAnalyzer (open-source)" for this. It is open source and ad-free. It is also worth looking at where the WiFi hub is located when you are doing the downloads.
This article might help.
Another common reason for disconnections is the device going to sleep and turning off or throttling WiFi.
This article might help.
As for the books and covers disappearing, sorry to say it but it sounds like the SD card you bought doesn't work, especially as it seems that the 64GB card did work. Perhaps the SD card file system (FAT32) is corrupt, perhaps the card advertises more space than it actually has (this is quite common on inexpensive cards), perhaps there is a bug in Samsung's SD card support, or perhaps the card simply doesn't work. In any case, files will disappear because the directory entries will be lost/corrupted.
Things to try:
- Did you format the new SD card in the Samsung device where it is used? That is a frequently recommended step, because sometimes the SD cards are formatted in a strange way or in something other than FAT32.
- If you have a card reader that you can attach to a desktop machine then you can use that machine to do a file system check of the card.
- Google the card to see if it is known to report more space than it actually has.
- Google "samsung 128gb sd card doesn't work". There are a lot of articles discussing this problem and things the users did to fix them.
- Put the old card back in. If things work again then you know where the problem is.