Adding a second My Passport Drive is not seen

I had a working 1TB My Passport Hard Drive for Mac and it was working fine. I bought a second drive, a 4TB My Passport Hard Drive for Mac and the second disk isn’t seen, even though the light comes on. If I connect the 4TB drive first, it is seen and operates fine, but then adding the known working 1TB drive, that isn’t now seen. Why am I unable to connect two drives to my Mac running High Sierra OS X 10.13.3?

I was going to buy Seagate, so may have to send the 4GB back for a refund as it doesn’t work and buy a Seagate.

Are you using any USB hub? if yes then try to use the drive without USB hub.

Both drives worked independently but being able to use them with the hub was essential or they were not fit for purpose. As it turned out, following a suggestion by my son, fully powering down the MacBook waiting then powering it up, reinitialised the USB subsystem and it came up faultlessly. Using multiple restarts had not achieved this result.