Should you use for your Samsung Galaxy S6 a "OTG" cable, then it can happen that your smartphone does not recognize correctly the connected USB stick or memory card.
For all who do not know the concept of OTG. OTG stands for "On The Go" and is a quick and easy option to move data directly from your smartphone to a USB stick or a memory card. OTG cable can be bought for example here.
So if your Samsung Galaxy S6 does not recognize the connected USB drive or your memory card using the OTG cable, then please try the following tip:
Unfortunately, the operating system does not recognize formatting that have been made with the NTFS file system. Only the FAT32 file system is recognized properly and reliably by Android. Therefore, you should format the USB stick or memory card with FAT32. This is best done via the PC.
To format a USB stick with FAT32 in Windows please proceed as follows
1. Stick your storage device to the USB port of the PC
2. Click on "Computer" or "My Computer" and then right click on the newly added removable disk
3. Select within the pop-up menu "Formatting".
4. In the window that opens, change "File System" in "FAT32"
5. Formatting the USB stick or memory card with the file system (Attention, all data will be deleted)
Then you can connect the USB flash drive again with the OTG cable and your Samsung Galaxy S6. This should now detect the disk properly and allow access via a File Explorer. Should that not be the case, then it may be that your USB Stick requires an additional power supply. Check that by proofing the specifications of your USB stick or external hard drive.