If I'm not mistaken, (been a while since I've touched my m7 and things have changed since on newer HTC devices) ...
If the proper fastboot drivers are installed on your PC, the device would read: "Fastboot USB", not just "Fastboot".
Which is what mode I believe you need to be in.
Sent from my HTC6545LVW using Tapatalk
PS C:\ADB\platform-tools> ./fastboot devices
PS C:\ADB\platform-tools> ./fastboot erase modem_st1
******** Did you mean to fastboot format this ext4 partition?
Erasing 'modem_st1' FAILED (remote: not allowed)
Finished. Total time: 0.016s
There has to be something I'm missing.