Reformatted Windows HDD shows up twice in Disk Utility Had a 1TB hard drive in a Windows laptop that I replaced with an SSD. I've put the old hard drive in a USB enclosure so I can use it as an external drive.
I plugged it into my Mac and opened disk utility where I saw two new mounted drives:
WINDOWS
RECOVERY

I clicked on the WINDOWS drive (981.9GB) and erased it to the ExFat format.
However, the RECOVERY drive is still there (16.85GB).
I've tried unplugging it, using the MacOS format, etc. but they still show up as separate drives.
How can I merge these into one external hard drive?
They aren’t two separate drives, they are two different partitions on the same drive.  You need to repartition the drive so that there’s only one partition.
If you issue the command:
% diskutil partitionDisk /dev/diskX 1 GPT JHFS+ “New Name” 100%

Will wipe the entire drive with a single partition using a GPT partition scheme and format it with the Journaled HSF+ (macOS) format.
If you issue the command diskutil partitionDisk without any arguments, you will get the following help info:
diskutil partitionDisk MountPoint|DiskIdentifier|DeviceNode
        [numberOfPartitions] [APM[Format]|MBR[Format]|GPT[Format]]
        [part1Format part1Name part1Size part2Format part2Name part2Size
         part3Format part3Name part3Size ...]