Transferring data from an external ssd to to hd
If I transfer all my data from an external ssd to my HD on my iMac, what's going to happen to all my data which is already on my HD? I would prefer to have only all my new data there. Should I erase the old data from my HD first?
Please advise!
Thank you!
iMac 21.5″, macOS 10.13