I have taken the memory cards out of my cameras and put them in the memory card slots in my HP a6152n computer many times. They work like disk drives - you can see the files and copy them from the memory cards to the computer.
However last week, the letters associated with the memory card slots - letters like R:, S:, T: and U: vanished from my machine. I used HP's chat and it was clear that the System Devices did not show the memory card slot hardware. One might have thought a wire was loose.
As I had not opened up my computer nor moved it I thought it very unlikely that a wire could come loose. So I waited and saw what would happen.
Then I switched on my printer, an Epson R300, and it was taking up drive letter S:. I rebooted my computer and switched off the printer. The memory device letters reappeared when I listed the disk drives on my computer. So I rebooted again, this time with the printer turned on and changed its disk letter to O using the Disk Management for Storage - that comes under the Administrative Tool, Computer Management.
I then rebooted the computer and all the drive letters were present. So I didn't have to open up the computer to connect the wires. The HP technician, while extremely helpful, never considered this possibility - neither did I. Hopefully I can save you time if your drive letters magically disappear by this post!