thunderbolt 2 drive not showing on my iMac

I have an iMac (Retina 5K, 27-inch, Late 2015) running BigSur 11.4

I also have a Lacie Rugged drive with a built in thunderbolt 2 cable.

I used to be able to plug in the drive using the thunderbolt 2 cable, but now the drive does not mount (it does not appear in Disk Utility either). The drive has a second usb port. If I use that port then the drive does mount.

I do not have a second drive to test the thunderbolt 2 port on on my iMac, but I do plug a screen in using a thunderbolt to DVI adaptor and that works fine.

If I plug the drive into my macbook pro using the thunderbolt 2 then that works fine.

Does anyone have any experience of how I might be able to resolve this?

(I have a few drives and accessories connected using USB, so it would be helpful if I could use the thunderbolt instead.)

Thanks