I feel a little silly, but I cannot seem to get my Nexus 5 flashed.

So, I boot the phone into recovery (TWRP).

I choose "wipe" (default options, no changes).

I choose "install".
Then, I choose the two zip files,
a) the CM12.1 one suggested here
b) the 4.0.1.48 one linked here
and flash them.

Afterwards I reboot and then the phone just sits there with the Google logo indefinitely.

What am I doing wrong?

P.S. I'm getting one error message while flashing the 4.0.1.48 zip file:

Code:

mount: failed to mount /dev/block/platform/msm_sdcc.1/by-name/userdata at /data: Device or resource busy

Copying and running scripts seems to proceed regardless.

P.P.S. I can get rid of the error message by issuing a "umount /data" through adb shell before flashing the 4.0.1.48 zip file. Still caught at the Google logo. The LED does not come up at all.

P.P.P.S. When I flash cm13 and SF 3.0.3 everything works even though the same failed to mount /dev/block/platform/msm_sdcc.1/by-name/userdata at /data message comes up. Thus, I assume this is not the source of the problem. The two zip files mentioned above (CM12.1 and 4.0.1.48) just do not seem to create a working system. Am I the only one affected by this?