Hello,
To troubleshoot the issue further, split the backup job if job is very large.
Secondly, if you are including any folder(s) that are related to any databases (E.g.: Oracle, SQL) then please exclude the same from the backup job. This will relieve OFO to avoid create much larger static volume.
You may refer the following technote for resolution,
"Volume specified is not snappable. Possible reasons: Not a fixed drive, or contains an active cache file or does not have a valid source or is write protected" (a0008501 HEX or e0008501 HEX) is reported in the backup job log when backing up using the Advanced Open File Option.
http://seer.support.veritas.com/docs/257796.htm
If that does not help also refer this technote,
http://seer.support.veritas.com/docs/267656.htm
Thank you,
Shweta