514,000 ExFat picture files not compatable with Catalina. unlocked HD. my external location will remain incompatible and unusable till i find a transfer algorithm?

I’ve read only APSF’s are compatible on Catalina. You must erase and reformat a new drive to APSF’s to use. But how do I move the files to the new drive?

Read some more. You didn't read all of it. Or, you may have read some disinformation that someone created for you.

How is your drive connected to the Mac?

Does it have its own power supply? Many bus-powered drives attempt to pull too much current from the USB, causing it to shut down, and the drive doesn't boot.

macOS Catalina can read and write to a variety of differently formatted disks as I said earlier.

Ex-FAT is the format of the disk the picture files are on, the files themselves are not affected by the way the disk has been formatted.

Macs and macOS's including Catalina can read and write to Ex-FAT formatted disks. You should be able to drag and drop the files to and from your mac to the disk.

My HD is plugged into one of the USB's in the back of my iMac.

My WD HD has another power connection. Just the USB

I don't understand.

However, it doesn't matter, if your hard drive is only powered by the USB, then try a powered USB hub or get a Y-cable to power it and send data. Or, if it has an external power source, use that.