iMac: Seagate Backup Plus Desktop Drives behaving differently with a Thunderbolt 1 cable and microUSB cable

I have a Seagate Backup Plus Desktop Drive 3TB (from 2014-15) that i have been using since i bought it. That drive came with a microUSB cable. I have been using it with my iMac 2011 (yes VERY OLD) which has a Thunderbolt 1 port. So I opted to buy a Seagate Thunderbolt adapter so the data transfer would be faster.

Fast forward to today. I bought a new iMac 2020 thanks to Black Friday sale. And I ran Time Machine backup on my OLD iMac to transfer to the new iMac. And here are my issues.

The new iMac only has the thunderbolt 3/ USBC and does not have thunderbolt 1/2 port anymore. So I decided to connect the Seagate drive to the new iMac using the usb micro cable that came with the box originally.... and I am getting "unreadable" message from iMac - WTH???? I connected the Seagate drive back to the old iMac with Thunderbolt cable, and confirmed its all there. However, when i replaced the thunderbolt cable with the microUSB cable in the old iMac, I also see "unreadable".

UGHHHHHH - is that how those old Seagate drives crazy like that? They read differently with different data cables? Input appreciated!