You cannot connect a USB drive on a microSD card slot. The specifications and format for a microSD card and USB interface are totally different from each other. A USB has just 2 pairs of terminals, one for the power (the two outer ones) and one for the data (the two inner ones). A USB drive has an inbuilt interface chip with its own clock generating crystal and a flash memory chip. while a microSD card is just a flash memory without any supporting hardware. The supporting hardware for the memory card is on the host side. USB slots are more popular in devices because of the robustness of the devices and the USB protol and standard is a free to use platform while incorporating an memeory card interface will need licensing from its developers.