S-On, No OS, UNLOCKED, No TWRP and want to install custom OS on HTC One M8 I have a HTC One M9 and would like to install the cRDroid ROM. 
When going into the bootloader I see:


Software status: Modified
UNLOCKED
S-ON
(Now the options)


REBOOT
REBOOT TO BOOTLOADER
BOOT TO DOWNLOAD MODE
BOOT TO RECOVERY MODE
POWER DOWN



I have tried to flash the OS via fastboot fastboot flash boot boot.zip this returned:
FAILED (remote: cannot flash this partition in s-on state).
When I tried to boot with fastboot boot boot.zip I got:  

creating boot image...
creating boot image - 381388800 bytes
downloading 'boot.img'...
OKAY [ 12.064s]
booting...
FAILED (remote: dtb not found)


Also booting to download mode got me Failed to boot to download mode
The Problems turned out to be dead NAND flash, which will need to be replaced