transferring a few files from a 2009 iMac to new iMac.

Ahhh...OK, then.

All you'll need is a USB A<--> USB C adapter. This is one available from Apple: USB A to USB C Adapter

This adapter will allow you to connect the external drive to your new Mac.

You can get and use an Apple branded or a third party USB-C to USB-A adapter.

USB-C to USB Adapter - Apple

https://www.amazon.com/dp/B08Z768LCW/ref=sspa_dk_detail_1?

https://www.target.com/p/philips-usb-c-to-3-0-usb-a-female-adapter/-/A-79804275

Have you tried ejecting the external drive from your old Mac...connecting it to your new Mac...then navigate to where the photos are on the external drive...selecting them...then copying them to your new Mac?

First you need to plug the old external drive into the new Mac.

Then drag and drop the selected photos from the external drive to the new iMac.

Or optionally open the Photos App. on the new iMac and Import the selected photos into the Photos App.

If you are having a problem connecting the old external drive to the new iMac, then you may need a different cable or adapter.

The question at this point is, what Make and Model is the 1.5T External Drive???

You're welcome.

I have 4 of those Philips adapters and they work fine with the later Intel and new Silicone Mac's.