Hi - What do you mean by "I can't connect the device to my MacBook AIr because of the cable. I suspect it's because of USB-C" ?

I also don't totally understand what your set up is...but... I got this same LaCie disk to connect to my MacBook Air (~2012 or so) via the Thunderbolt port. I got a USB-C to Thunderbolt adapter and thought it was going to be smooth sailing but it turned out it is incompatible on a hardware level...as far as I remember. I can't find the references now but I'm pretty sure that was the problem. But, I had an old MacBook Air.