If I downgrade from Mavericks back to Snow Leopard and external HD's to be recognised?

I have had to reboot the system before after a problem but over the course of time since upgrading my iMac has been just unbearbly slow.

I have tried so many ram fixes but none are working. I am saving up for a new mac but in that time I will need this one to be functioning properly.

One thing I noticed when OSX re-install was some of my HD's were not recognised. Can this be altered?

Thanks