Leave your boot set to BIOS. Make sure you press f11 on boot. Go into C: drive then your USB will appear to boot from.
Now the reason it wont boot on DELL is because your syslinux is not in the image and is required for the Dell servers.
1. Create your USB stick image manually in a linux distro.
- Install pop OS on your spare laptop or PC or in hyperviser like VMWARE Workstation or Virtual Box. Make sure you are bridged on your Ethernet so pop OS can access the net on your existing network.
Boot up pop OS and install USB into your machine. Share the USB with the pop OS VM (different on each hypervisor but basically telling the hypervisor to pass through the USB stick to your pop OS install.
Open browser in pop OS and download the latest zip file of the UNRAID files from their website.
Once downloaded you can open "DISK" which is in the utility's menu on the GUI on pop OS
Click on the negative sign to delete any existing partitions on your USB stick
Then click on + and create a new partition
Then name the partition label to UNRAID in capitals
If the partition isn't formatted to FAT32 then format it using the same utility it's straight forward.
Now extract your zip using the GUI just lick on files, then click on downloads, then right click on your downloaded zip to extract "HERE"
Now you can select "al"l and COPY
Then you can open your USB folder and paste in the files, this may take a couple of minutes so be patient and wait for then folders to appear.
Ok now open terminal from the gui.
type the following
sudo apt install syslinux
after this is installed navigate to your downloads folder
If you type the command "ls" , this will list your folders.
then for example
type cd Downloads (Downloads has a CAPITAL "D" so make sure you type it correctly)
type "ls"
you should now see your unRaidServerxxxxx folder
xxxx being the current release
type "cd" unRaidserverxxxxxxx to change to this folder
type "ls" and you should see the contents of the folder
now type
sudo ./make_bootable_linux
if this doesn't work press the up arrow key and press enter this will execute the command again
select UEFI "no"
Thats it now your USB will boot on Dell T/R series servers in BIOS mode.