Posting this information here in case someone else runs into the same problem. I just bought an X651, and a CM4 with eMMC flash. I had a lot of trouble flashing the eMMC with my Linux PC. The process would always timeout and the kernel log would show USB bus resets. I tried several different power supplies, USB cables, and different PCs. What seemed to work for me was changing how I ran the rpiboot utility. Specifically following their readme worked:
git clone --recurse-submodules --shallow-submodules --depth=1 https://github.com/raspberrypi/usbboot
cd usbboot
make
sudo ./rpiboot -d mass-storage-gadget
On my earlier attempts, I had not done a recursive git clone, and I was only running 'sudo ./rpiboot'. After following the above instructions, the RPI imager showed the device as "mmcblk0 Raspberry Pi multi-function USB device" instead of "RPI-MSD -0001". This allowed me to flash the device quickly and reliably.
Hope this hels someone else.
