[Xiaomi Poco X3 NFC Questions & Answers] vjsa:
Safe to say I have messed up badly. This morning, I was trying to update EvolutionX build (by dirty flashing) and I received an error: Cannot mount /product (invalid argument). After few tries, I still couldn't flash anything and neither could I now boot into the system. So I decided to wipe the data as well as system partition and then try to install the ROM zip. It failed again, but now the error said that it didn't find the /system and /vendor as well.

From the TWRP Recovery > Mount section, I verified that I could not check the system, vendor, and product partitions. I could select the rest. Recovery seems to be working fine.

I checked the partitions on phone:

>adb shell cat /etc/fstab

/system_root ext4 ro 0 0

/vendor ext4 ro 0 0

/product ext4 ro 0 0

/dev/block/sda10 /metadata ext4 rw 0 0

/dev/block/sda16 /data f2fs rw 0 0

/dev/block/sda11 /cache ext4 rw 0 0

/dev/block/sde4 /firmware vfat rw 0 0

/dev/block/sda7 /cust ext4 rw 0 0

/dev/block/sda2 /persist ext4 rw 0 0

/usb_otg vfat rw 0 0

/dev/block/mmcblk0p1 /external_sd vfat rw 0 0

>adb shell cat /proc/partitions

major minor #blocks name

1 0 8192 ram0

1 1 8192 ram1

1 2 8192 ram2

1 3 8192 ram3

1 4 8192 ram4

1 5 8192 ram5

1 6 8192 ram6

1 7 8192 ram7

1 8 8192 ram8

1 9 8192 ram9

1 10 8192 ram10

1 11 8192 ram11

1 12 8192 ram12

1 13 8192 ram13

1 14 8192 ram14

1 15 8192 ram15

8 0 120479744 sda

8 1 8 sda1

8 2 32768 sda2

8 3 1024 sda3

8 4 512 sda4

8 5 512 sda5

8 6 131072 sda6

8 7 1048576 sda7

8 8 131072 sda8

8 9 64 sda9

8 10 16384 sda10

8 11 393216 sda11

8 12 393216 sda12

8 13 16384 sda13

8 14 8192 sda14

8 15 8388608 sda15

259 0 109918092 sda16

8 16 8192 sdb

8 17 7168 sdb1

8 18 128 sdb2

8 32 8192 sdc

8 33 7168 sdc1

8 34 128 sdc2

8 64 4194304 sde

8 65 512 sde1

8 66 4096 sde2

8 67 512 sde3

8 68 262144 sde4

8 69 1024 sde5

8 70 4096 sde6

8 71 32768 sde7

8 72 8192 sde8

8 73 32768 sde9

8 74 512 sde10

8 75 512 sde11

8 76 512 sde12

8 77 128 sde13

8 78 64 sde14

8 79 2048 sde15

259 1 2048 sde16

259 2 512 sde17

259 3 4096 sde18

259 4 512 sde19

259 5 4096 sde20

259 6 32768 sde21

259 7 8192 sde22

259 8 512 sde23

259 9 512 sde24

259 10 512 sde25

259 11 2048 sde26

259 12 2048 sde27

259 13 4 sde28

259 14 1024 sde29

259 15 256 sde30

259 16 8192 sde31

259 17 33424 sde32

259 18 4 sde33

259 19 1024 sde34

259 20 8192 sde35

259 21 2048 sde36

259 22 65536 sde37

259 23 128 sde38

259 24 32 sde39

259 25 32 sde40

259 26 512 sde41

259 27 28 sde42

259 28 512 sde43

259 29 1024 sde44

259 30 64 sde45

259 31 131072 sde46

259 32 24576 sde47

8 48 131072 sdd

8 49 104 sdd1

8 50 128 sdd2

8 51 1024 sdd3

8 80 131072 sdf

8 81 104 sdf1

8 82 2048 sdf2

8 83 2048 sdf3

8 84 10240 sdf4

8 85 128 sdf5

179 0 7761920 mmcblk0

179 1 7757824 mmcblk0p1

It seems that system_root, vendor and product do not have any block (?) assigned to them. Is there a way to identify what were the original assigned blocks to these partitions and assign them through fastboot/adb/recovery?

Any help is appreciated thanks!!

UPDATE: Flashed the fastboot rom and it solved the issue. Got the stock MIUI and flashed the custom recovery/rom from there on. See last reply for details.