RMAN-06056: could not access datafile 4

This error is occur when RMAN is not able to access the desired datafile.

Starting backup at 05-AUG-11

using target database control file instead of recovery catalog

allocated channel: ORA_DISK_1

channel ORA_DISK_1: sid=147 devtype=DISK

allocated channel: ORA_DISK_2

channel ORA_DISK_2: sid=154 devtype=DISK

could not read file header for datafile 4 error reason 1

RMAN-00571: ===========================================================

RMAN-00569: =============== ERROR MESSAGE STACK FOLLOWS ===============

RMAN-00571: ===========================================================

RMAN-03002: failure of backup command at 08/05/2011 11:39:08

RMAN-06056: could not access datafile 4

Solution: You can instruct RMAN in this case to skip this datafile

RMAN> backup database skip inaccessible;

After this everything  will go fine..

Enjoy:-)