redhat partition disapears after reboot

question:

creating partition, and running mkfs.ext3.

everything seems ok.

but After reboot the partition is gone:

[root@tstr ~]# fdisk /dev/sdc3

Device contains neither a valid DOS partition table, nor Sun, SGI or OSF disklabel

Building a new DOS disklabel. Changes will remain in memory only,

until you decide to write them. After that, of course, the previous

content won't be recoverable.

Warning: invalid flag 0x0000 of partition table 4 will be corrected by w(rite)

Command (m for help): p

Disk /dev/sdc3: 6440 MB, 6440361984 bytes

255 heads, 63 sectors/track, 782 cylinders

Units = cylinders of 16065 * 512 = 8225280 bytes

Device Boot Start End Blocks Id System