Which model PI?
What are you using for a power supply?
What kind of drive are you attempting to load the OS from?
For systems not capable of true USB boot, the simplest solution is to clone the entire card to the USB drive (preferably with the SD Card Copier utility included with Desktop versions of Raspbian), then copy just the bootcode.bin file to a FAT32 formatted SD card. That will load and run the entire OS from the USB drive on all models up to the 3B+ (although the 3B+ can USB boot without a card as well).
The new Pi 4B does not currently support USB booting, and does not use the bootcode.bin file, so you do need to have /boot on the card (for now). In that case, clone the card to the USB drive, then copy the contents of the "boot" partition to a FAT32 formatted micro SD card. After the system boots, edit /etc/fstab to mount the SD card as /boot (followed by a reboot) so that firmware and kernel updates are properly applied.
Forum member RonR has some handy scripts that can simplify the process.
My mind is like a browser. 27 tabs are open, 9 aren't responding,
lots of pop-ups...and where is that annoying music coming from?