This is a self answered question that is different and may save hours of frustration to somebody
Related questions but none that helped
- ADB detects my device but fastboot doesn't
- Yureka fastboot not detected in LINUX, but adb detects device
- ADB detects but, fastboot does not detect device in windows 10
- adb can find device but fastboot cannot
- fastboot not showing device (nokia 2.1 ) but adb works fine
- adb devices show phone in debug mod but not in fastboot mod
- Fastboot detects phone in the fastboot mode but not when phone is on
After more than 6 months I thought of updating security patches on my Pixel 4a Android 11 (stock) and root it again.
I uninstalled Magisk , applied OTA update , extracted boot image and patched it with magisk (see this). All smooth
I needed to run
fastboot flash boot patched.img to root the device. But when I run the command response is
<waiting for any device>, which was strange since
adb devices shows device serial.
Things tried, without any luck
Updated adb on Lint Minux, it was latest
Switched to Windows 10 and updated
Andrew in chat pointed to an XDA thread, which was better covered by Izzy here How do I get my device detected by ADB on Linux?. Added udev rule but then no dice.
Firelord in chat suggested it could be
plugdevissue so tried the solution mentioned in this Stack Overflow question adb devices => no permissions (user in plugdev group; are your udev rules wrong?), though the problem wasn't similar
Multiple cables , rebooting Android, Windows, Linux when I was at a loss to figure out
Variations of fastboot like
fastboot flash boot_a patched.img,
fastboot flash boot --slot all patched.imgand
sudo fastboot devices
Finally, the solution ....