XPS 8940 doesn't have a Legacy boot option and something doesn't add up here... Are  you sure you have a 280 GB Optane SSD? The specs say the supported Optane SSD is only 32 GB. Where did you find that Intel Optane firmware update? An Intel Optane SSD isn't used as a boot drive. Optane is used as a cache to speed up performance of a slow HDD, and rarely to speed up an SSD.  So is your boot drive (C:) an HDD or an SSD? Go to the Support page and enter your Service Tag (don't post it here). When your system is recognized, look under Quick Links on right side of page for a link to View Product Specs.  Look down the list for an entry saying something like:  PCIe x4 SSD and/or 1TB 7200RPM HDD (yours may be somewhat different).  Expand that entry and see exactly which drives are is installed in your PC. An SSD might be from Samsung, WDC, Kioxia, Micron, SSSTC, etc, and/or one of various HDD brands. Making an ISO on DVD is not how you would install an SSD firmware update, assuming you actually need one. You'd just download the .exe file onto your PC.  Double-click the download and it will run the installation. Be good idea to back up (image) all files on the drive before running a firmware update, to be safe.
Am a happy user of XPS8940, with Intel 280GB OPTANE SSD. I found that the firmware of the OPTANE is rather old.  So, downloaded latest FW package and made ISO disk.  Read the user's guide and found that the disk must be booted in legacy BIOS mode.  But I could not find the legacy mode setting.   How can I do it? Tak
Optane Firmware update requires legacy BIOS, XPS8940