If you've used the drive on a system with legacy mode enabled, it's likely an MBR drive - which cannot boot a UEFI system. You'll need to prepare a new bootable drive with UEFI mode. If it's a Windows install you are doing, use the Microsoft Media Creation tool.  If it's some other type of bootable drive, you'll need to consult the documentation for that software on how to prepare a drive that's UEFI-bootable. [URL]
I have a bootable USB flash drive that is not recognized by dell Vostro 3400. legacy boot is not supported on this device. please suggest any other solution...
bootable USB drive not recognized by BIOS Support in Vostro Dell Laptop Windows 
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