Postgres not being able to create a tablespace

I just mounted an external HD for store my new tablespace.

postgres@vostro ~ $ df -k
Filesystem     1K-blocks     Used Available Use% Mounted on
/dev/sdb1      480589520    71580 456082280   1% /mnt/DB

postgres@vostro ~ $ ls -l /mnt/DB/
total 4
drwxrwxrwx 2 postgres postgres 4096 May 25 17:02 postgres

demisc@vostro /mnt $ mount | grep -i sdb1
/dev/sdb1 on /mnt/DB type ext4 (rw,nosuid,nodev,relatime,data=ordered)

I gave all the necessary permissions so that posgres could create the new ts in /mnt/DB/postgres. Here is the user groups:

postgres@vostro ~ $ id
uid=123(postgres) gid=131(postgres) groups=131(postgres),112(ssl-cert),1000(demisc)

Happens that when running the create tablespace command I get the following:

postgres=# create tablespace ts_project location '/mnt/DB/postgres';
ERROR:  could not set permissions on directory "/mnt/DB/postgres": 
Permission denied

What I am missing?

ls -ln /mnt/DB/postgres

returns:

demisc@vostro /mnt/DB $ ls -ln /mnt/DB/postgres/
total 0

Everything looks good. There is no reason why postgres user can not create the tablespace. If i switch to postgres and try to create file or dir in the path it works fine.

I just found a solution. I changed the permission of /mnt/DB/postgres to 700:

postgres@vostro /mnt/DB $ ls -l
total 4
drwx------ 3 postgres postgres 4096 May 25 17:46 postgres

Now it worked the creation of the ts

postgres@vostro /mnt/DB/postgres $ ls -l
total 4
drwx------ 2 postgres postgres 4096 May 25 17:46 PG_9.5_201510051

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StackExchange Database Administrators Q#174615, answer score: 3