Symptoms:
ORA-19502: write error on file "/opt/oracle/XE/backupset/2017_02_21/o1_mf_annnn_TAG20170221T095912_dbr0c6f8_.bkp", block number 2721792 (block size=512)
ORA-27072: File I/O error
Cause:
There is not enough disk space on the drive to complete the backup piece.
#df -kh|grep opt
Solution:
Cleanup unnecessary files on the drive or add more disk space on the drive.