Transfer data from one external hard drive to another

I would do it drive to drive. Transferring to iCloud will be slow so minimize transfers using Internet.

Just connect both drives to the computer and drag files from one to the other. Some modifications to the method might help depending upon what you are transferring and your objectives. For example, I have clones of my entire drives (including boot systems) so I use specialized software for doing that.

I am not as familiar with using iCloud drive. Refer to Add your Desktop and Documents files to iCloud Drive - Apple Support

Finder can open more than one window at a time to make copying files folder-to folder or drive-to-drive really easy.

Open a window with files and folders you want to see.

Choose New Window from the Finder's File Menu.

a new window opens.

resize the two windows and place side-by-side.

Navigate to view any other folder on any other drive.

compare contents easily, drag files back and forth, to get the results you desire.

I have 4 drives connected to my 2012 iMac, one being a boot drive. So that's 5 if you include the internal and 6 if you include the occasional thumb drive. :-)

The main limit for most users will be with newer Macs they have really trimmed down ports. You can still use multiple drives but they are sharing the same port (albeit very fast ones) and then you have to invest in a hub.

do not worry in you have to resort to connecting one of your drives using USB-2.

USB-2 is fast enough for all but the very fastest (under 1 percent) of all Rotating Magnetic drives, and only causes an undetectably small delay for that top 1 percent.

if you get it right, both drives will appear as drives in Finder.

My 2012 iMac has 4 USB3 ports. I also have a 10 port USB2 hub. have a DVD drive plugged into the hub. I have a dual docking station and one single docking station plugged into the iMac ports and one single docking station plugged into the hub. I infact have more than 4 hard drives, and the docking station lets me swap them in a matter of seconds vs. actual enclosures. I can also keep a fan blowing over them in summer so they stay nice and cool.