nbd-client Fsync error

i try insert USB memory and check the device is "/dev/sdb1"

root@ubuntu:~# lsblk

NAME MAJ:MIN RM SIZE RO TYPE MOUNTPOINT

sda 8:0 0 64G 0 disk

├─sda1 8:1 0 63G 0 part /

├─sda2 8:2 0 1K 0 part

└─sda5 8:5 0 1022M 0 part [SWAP]

sdb 8:16 1 3.8G 0 disk

└─sdb1 8:17 1 3.8G 0 part /media/fd74eeb0-2bf8-49b0-872a- 4a283f124c10

sr0 11:0 1 1024M 0 rom

sr1 11:1 1 1024M 0 rom

and create network device

nbd-server -r 2000 /dev/sdb

** (process:2778): WARNING **: Specifying an export on the command line is deprecated.

** (process:2778): WARNING **: Please use a configuration file instead.

and client side( same machine )

nbd-client 172.20.10.3 2000 /dev/nbd0

Negotiation: ..size = 3850MB

bs=1024, sz=4037017600 bytes

and mount nbd device

mount -o loop /dev/nbd0p1 point/

but mount as follows problem occurs

root@ubuntu# mount /dev/nbd0p1 point/

mount: block device /dev/nbd0p1 is write-protected, mounting read-only

when i tried modify files in device error occurrs

vi point/test.c

add lines

and :wq

point/test.c" E212: Can't open file for writing

why ??

i don't understand this

root@ubuntu:~# dmesg | tail

[ 371.280809] loop: Write error at byte offset 3035136, length 4096.

[ 371.280810] Buffer I/O error on device loop0, logical block 741

[ 371.280812] lost page write due to I/O error on loop0

[ 371.280815] loop: Write error at byte offset 4096, length 4096.

[ 371.280816] Buffer I/O error on device loop0, logical block 1

[ 371.280818] lost page write due to I/O error on loop0

[ 389.039901] loop: Write error at byte offset 8396800, length 4096.

[ 389.039909] EXT4-fs warning (device loop0): ext4_end_bio:317: I/O error -1 writing to inode 12 (offset 0 size 0 starting block 2050)

[ 389.039913] Buffer I/O error on device loop0, logical block 2050

[ 391.266099] hrtimer: interrupt took 8643276 ns