I have a new iMac and can't see a way to connect my external hard drive

If your external drive has a USB connector (most do) you can use a USB to USB-C adapter such USB-C to USB Adapter - Apple

or search Amazon or other online for "USB to USB-C adapter."

Can't see the right port? You would need an adaptor or a new cable.

The small oval ports on the back are USB-C and support USB4, Thunderbolt, and DisplayPort.

If you have an USB-A plug on your drive, you can get a USB-A to USB-C adaptor, or get a new cable that plugs into your drive and has a USB-C connector at the other end.

https://www.amazon.com/s?k=usb-c+to+a+adapter&adgrpid=1341404750473834&hvadid=83837868003564&hvbmt=be&hvdev=c&hvlocphy=74005&hvnetw=o&hvqmt=e&hvtargid=kwd-83838143027425%3Aloc-190&hydadcr=18008_10500489&tag=mh0b-20&ref=pd_sl_ysagolsy1_e