If you do not want to erase the flash drive, then follow these steps.
Open the Disk Utility.
From the View pulldown menu on the Disk Utility main window, select Show All Devices.
Highlight the flash drive.
Note the Device. You should see diskN were N is a integer.
In a Terminal application window, enter the following command. Replace N with the integer from the previous step.
sudo diskutil mount diskNs1
Note the name of the mounted volume. Here, I will assume the name was EFI. If you get a different name, make the appropriate substitution when entering the command given below.
Hold the option key when you power on and you can choose the working mac partition or insert a bootable DVD to get back to disk utility or a working os. This is called the Startup Manager screen.
The eject key will work once the Startup Manager is running so you can eject the windows DVD.
I think the problem is the way the computer is locked, if your computer has a Super Drive, your Mac is automatically configured/locked to boot from that drive and ignore the USB.
Their is no amount of reconfiguration or software tricks that would change that, what you can do is take out the Super Drive, or try cleaning it with a CD cleaner - the one with the brush.
Best Answer
Here, I assume the flash drive is the only external drive plugged into your Mac.
Use the Disk Utility to erase the flash drive. Use the options shown below.
Next, transfer the Catalina files to the flash drive.
If you do not want to erase the flash drive, then follow these steps.
Show All Devices
.diskN
wereN
is a integer.In a Terminal application window, enter the following command. Replace
N
with the integer from the previous step.Note the name of the mounted volume. Here, I will assume the name was
EFI
. If you get a different name, make the appropriate substitution when entering the command given below.Enter the following commands.