I've rooted my Nexus 7 using the SO instructions How do I root my Nexus 7?. I'm trying to install LineageOS using its instructions https://wiki.lineageos.org/devices/flo/install#installing-lineageos-from-recovery. But steps 4 and 6 fail (making a backup, and wipe / advanced wipe). Both failures include the messages Failed to mount '/data' (Invalid argument) and Unable to mount storage.
So /data may be corrupt.
From adb shell, ls /data shows that it's an empty directory; mount /data fails with the same error Invalid argument.
From adb shell, the Nexus 7's /etc/fstab mentions /dev/block/mmcblk0p30 /data ext4 rw 0 0. But when I try to repair that device with e2fsck, it finds no superblock, even with -b 8193.
From adb reboot bootloader, the screen shows SECURE BOOT - enabled and LOCK STATE - unlocked. From that fastboot screen, if I select recovery mode, I see the teamwin TWRP 3.1.1-0 splash screen for 30 seconds, then I "swipe to allow modifications," and then I see TWRP's 8 big buttons.
Well, the backup and wipe buttons fail, so just for giggles, I tapped the button Install (LineageOS steps 7-10), and chose the lineage and open_gapps .zip files that I'd adb pushed to /sdcard. To prevent the install from working, checked the checkbox "verify zip signature" without copying over the md5 files containing the signatures. Besides the expected failure to verify the signatures, I saw these errors:
Failed to mount '/data' (Invalid argument)
Unable to recreate /data/media folder
E:Could not create /cache/recovery/
Unable to mount storage.
Can't open destination log file: '/cache/recovery/log'
Unable to mount /data/media/TWRP/.twrps
E:DataManager::Output_Version -- Unable to make /cache/recovery
Again, /data seems to be the problem. I probably shouldn't attempt a real install before taking care of that.
How should I recover from these errors, to install LineageOS or, failing that, to restore the device to factory defaults? (If it helps, I have another Nexus 7, unrooted.)