An A2000 is best expanded internally. As far as external options, that depends if your SCSI controller has a port on the back for connecting external peripherals. Most (like the A2091) do. You'd have to mount a hard drive or other drive in some sort of external chassis to provide power, and configure it that way.
TL;DR. How old is that 50MB hard drive? If you value your data you should really think about replacing it with something more modern.
http://amiga.resource.cx/expansion.htmlhttp://www.bigbookofamigahardware.com/
