unfortunately I don't have enough space in my main hd too... do you believe I can still go to the bitcoin core settings and change path (to external hd) ?
That's possible, but I wouldn't do it: external disks sometimes disconnect when you touch the cable. That would corrupt your block data.
Taking a step back: what's the reason you want to run a full Bitcoin node? If you have neither the required disk space, nor the bandwidth, why not use a light wallet such as Electrum? Or solve the disk space problem by pruning the blockchain, unless you have a reason to need the full chain.