We got the following error when backing up an 11.2.0.3 database to an external disk.

RMAN> run  
2> {  
3> allocate channel d1_disk_backup device type disk   ;  
4> crosscheck backup;  
5> backup incremental level 1 cumulative as COMPRESSED BACKUPSET copies=1 tag 'DAILY_BCK' database format '/external_usb/mesutt_daily_%U.bkp';  
6> backup  tag '%ARCTAG' archivelog all not backed up since time 'SYSDATE-5' format '/external_usb/mesutt_arc_%U.bkp';  
7> backup  current controlfile tag 'CTL' format '/external_usb/mesutt_cntl_%U.bkp';   
8> backup  spfile tag 'SPF' format '/external_usb/mesutt_spfile_%U.bkp';  
9> release channel d1_disk_backup;  
10> }  
allocated channel: d1_disk_backup  
channel d1_disk_backup: SID=576 device type=DISK  
...  
....  
released channel: d1_disk_backup  
RMAN-00571: ===========================================================  
RMAN-00569: =============== ERROR MESSAGE STACK FOLLOWS ===============  
RMAN-00571: ===========================================================  
RMAN-03009: failure of backup command on d1_disk_backup channel at 10/06/2014 03:55:55  
ORA-19502: write error on file "/external_usb/mesutt_arc_pvpkbj7m_1_1.bkp", block number 16388096 (block size=512)  
ORA-27072: File I/O error  
Additional information: 4  
Additional information: 16388096  
Additional information: 339968

Let s check user permissions to this mount point,

vmuttora1(mesutt):/home/oracle>df -h  
Filesystem            Size  Used Avail Use% Mounted on  
/dev/mapper/VolGroup-lv_root  
                       36G   27G  6.8G  80% /  
tmpfs                  16G  276M   16G   2% /dev/shm  
/dev/sda1             485M   59M  402M  13% /boot  
/dev/mapper/vg01-jv01  
                       32G   18G   13G  58% /jv01  
/dev/mapper/vg02-rman  
                      197G   17G  171G   9% /rman  
vmuttora1(mesutt):/home/oracle>

The disk is not showing; /external_usb/ not mounted. Let s mount it and give oracle user permission to write on it.

[root@vmuttora1 external_usb]# mount /dev/sde1 /external_usb/  
[root@vmuttora1 external_usb]# chown oracle:dba /external_usb/  
[root@vmuttora1 external_usb]# df -h  
Filesystem            Size  Used Avail Use% Mounted on  
/dev/mapper/VolGroup-lv_root  
                       36G   27G  6.8G  80% /  
tmpfs                  16G  276M   16G   2% /dev/shm  
/dev/sda1             485M   59M  402M  13% /boot  
/dev/mapper/vg01-jv01  
                       32G   18G   13G  58% /jv01  
/dev/mapper/vg02-rman  
                      197G   17G  171G   9% /rman  
/dev/sde1             1.8T   74G  1.7T   5% /external_usb

Re-run the backup script and the error is gone.
to keep it permanent, add the reference to the device to /etc/fstab;

[root@vmuttora1 external_usb]# blkid /dev/sde1  
/dev/sde1: UUID="0df0b87c-7569-4eba-90b4-4cd8e7711421" TYPE="ext4"  
[root@vmuttora1 external_usb]# tail -n 1 /etc/fstab  
UUID=0df0b87c-7569-4eba-90b4-4cd8e7711421 /external_usb                ext4    defaults        0 0