Rman backup

I created rman script in RHEL4AS for daily backup. If we run the script manually in command prompt it will run ,if the load is very high. But on the time it is starting ,but after a long time it'll terminate and returning an error like following...
ORA-27037: unable to obtain file status
Linux Error: 2: No such file or directory
Additional information: 4
My log file shows like this

RMAN-03009: failure of backup command on ORA_DISK_4 channel at 08/27/2006 08:58:
49
ORA-27052: unable to flush file data
Linux Error: 28: No space left on device
Additional information: 1

Recovery Manager complete.
formation: 1

Recovery Manager complete.
unable to flush file data
Linux Error: 28: No space left on device
Additional information: 1

Recovery Manager complete.

but space is moerthan enough .
pls explain the reason and solution as soon as possible
regards
Lovy Ismail
Oracle DBA
Multichoice Middle East