[Motorola Photon Q 4G LTE] nemo.:
I tried performing the mod, got as far as to test boot my previously (multi-reboot lasting) CM installation but got no further than the CM boot screen.

Recovery works and the last_kmsg indicates that the flash is bad (bad blocks) or at least that there are connection issues:

[ 105.525957,0] mmcblk0: error -110 transferring data, sector 5615320, nr 8, cmd response 0x900, card status 0x0
[ 105.526506,0] end_request: I/O error, dev mmcblk0, sector 5615320
[ 105.927544,0] mmc0: CMD17: Data timeout. DAT0 => 1
[ 105.927910,0] mmc0: SDCC PWR is ON
[ 105.928093,0] mmc0: SDCC clks are ON, MCLK rate=96000000
[ 105.928307,0] mmc0: SDCC irq is enabled
[ 105.928643,0] ===== SDCC-CORE: Register Dumps @base=0xe18b2000 =====
[ 105.928856,0] Reg=0x00: 0x00000001, 0x0100df00, 0x0055aed9, 0x00001451
[ 105.929192,0] Reg=0x10: 0x00000011, 0x00080900, 0x0f5903ff, 0xffffffff
[ 105.929406,0] Reg=0x20: 0x8a40407e, 0x0124f800, 0x00000200, 0x0010200b
[ 105.929772,0] Reg=0x30: 0x00000100, 0x004c0400, 0x00000000, 0x408001ff
[ 105.930108,0] Reg=0x40: 0x00000000, 0x00000000, 0x00000000, 0x00000000
[ 105.930321,0] Reg=0x50: 0x00000018, 0x00000000, 0x00000000, 0x00000000
[ 105.930688,0] Reg=0x60: 0x60006400, 0x00000000, 0x00000000, 0x00000000
[ 105.930871,0] mmc0: MCI_TEST_INPUT = 0x000003fe
[ 105.931207,0] mmc0: SPS mode: busy=1
[ 105.931390,0] mmc0: xfer_size=512, data_xfered=0, xfer_remain=512
[ 105.931756,0] mmc0: got_dataend=0, prog_enable=0, wait_for_auto_prog_done=0, got_auto_prog_done=0
[ 105.932183,0] mmcblk0: r/w command failed, status = 0x80900
[ 105.932702,0] end_request: I/O error, dev mmcblk0, sector 5615321
[ 105.946741,0] sps:REVISION of BAM 0xe1810000 is 0x5.
[ 106.333923,0] mmc0: CMD18: Data timeout. DAT0 => 1
[ 106.334014,0] mmc0: SDCC PWR is ON
[ 106.334137,0] mmc0: SDCC clks are ON, MCLK rate=96000000
[ 106.334228,0] mmc0: SDCC irq is enabled
[ 106.334320,0] ===== SDCC-CORE: Register Dumps @base=0xe18b2000 =====
[ 106.334472,0] Reg=0x00: 0x00000001, 0x0100df00, 0x0055aed8, 0x00001452
[ 106.334594,0] Reg=0x10: 0x00000012, 0x00000900, 0x0f5903ff, 0xffffffff
[ 106.334686,0] Reg=0x20: 0x8a40407e, 0x0124f800, 0x00001000, 0x0010200b
[ 106.334838,0] Reg=0x30: 0x00000800, 0x004c0400, 0x00000000, 0x408001ff
[ 106.334930,0] Reg=0x40: 0x00000000, 0x00000000, 0x00000000, 0x00000000
[ 106.335083,0] Reg=0x50: 0x00000018, 0x00000000, 0x00000000, 0x00000000
[ 106.335174,0] Reg=0x60: 0x60006400, 0x00000000, 0x00000000, 0x00000000
[ 106.335296,0] mmc0: MCI_TEST_INPUT = 0x000003fe
[ 106.335388,0] mmc0: SPS mode: busy=1
[ 106.335540,0] mmc0: xfer_size=4096, data_xfered=0, xfer_remain=4096
[ 106.335632,0] mmc0: got_dataend=0, prog_enable=0, wait_for_auto_prog_done=0, got_auto_prog_done=0
[ 106.335998,0] mmcblk0: error -110 transferring data, sector 5615320, nr 8, cmd response 0x900, card status 0x80b00
[ 106.336151,0] mmcblk0: retrying using single block read
[ 106.347932,0] resout_irq_handler PMIC Initiated shutdown

So I booted into recovery and tried backing up /system, /cache and /data to /sdcard (using dd and TWRP backup). /data is the only one that fails with I/O errors. I also do not understand how /sdcard and /data can both be /dev/block/mmcblk0p39 (according to /proc/mounts). Does anyone have an idea what is causing this and what I could do to fix this?

Since the flash is almost directly below of the soldered SIM IC I speculated whether I accidently unsoldered some BGA pins of the flash IC but then I would not be able to backup /system to /sdcard with 12MB/s... I'm so confused right now :/