I have a harddisk which has 72G size , I want to mount it to path /ora_dir , I try to mount the device from sdd1 to sdd5 , but even I try all dev path , it still found the size is not 72G , could advise how can I normally mount the path ? thx

try 1 )
#mount /dev/sdd1 /ora_dir
df
#/dev/sdd1 5036224 3480872 1299524 73% /ora_dir

try 2 )
#mount /dev/sdd2 /ora_dir
df
#/dev/sdd2 5036224 4072364 708032 86% /ora_dir

try 3 )
#mount /dev/sdd3 /ora_dir
df
#/dev/sdd3 4032000 3736716 90464 98% /ora_dir

try 4 )
#mount /dev/sdd4 /ora_dir
df
#/dev/sdd4 2355868 803156 1433036 36% /ora_dir

try 5 )
#mount /dev/sdd5 /ora_dir
df
#/dev/sdd5 2016016 69432 1844172 4% /ora_dir

sdd1, sdd2, sdd3 and so on are DIFFERENT partitions of the drive...so the sum of the sizes of them should give you 72G.
Try the command cfdisk /dev/sdd to see them.