I have a slot on my laptop which I don't know what it's use is.
I have two slots for RAM, both of which are being used. I hoped to add more in the third slot but noticed it is smaller than the RAM slots. 9mm in width to be precise. It also looks to be square as opposed to rectangle. There is also a small thin cable cellotaped to the empty space.
I went to a computer fayre and was told it may be a space for a modem. I have an inbuilt modem already so wasn't totally confident of the advice.
Anything that is classified as SCSI. SCSI is a daisychain type connection. So for example you could have a SCSI printer and it would have two connectors on the back. One connects to the PC and the other can be used to connect another SCSI device and so on up to about (I think) 27 other devices. It is not used very often these days because USB will do almost the same thing. I have got a SCSI hard drive but because I don't have any SCSI cards in my computer I can't use it. This is a very old hard drive but you might be able to find a modern SCSI hard drive that you could use as an external extra drive. You will also find that most SCSI driven extras are more expensive. Each SCSI device that you have connected must be given an exclusive ID. So, the first one on would be 1, the second 2 etc..