partition table problem

Linux came preinstalled with formatted only half of a disk.
I run fdisk. it allows me to create partitions on the rest of a disk, but when I try to write this info on disk getting error:

 Command (m for help): w
 The partition table has been altered!

 Calling ioctl() to re-read partition table.
 Syncing disks.
 Re-read table failed with error 16: Device or resource busy.
 Reboot your system to ensure the partition table is updated.

I reboot my machine and all my new patitions disappear.
Please advice.
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well I guess it s because modifying the partition table of the disk where you ve got your root fs is not recommended.
Probably booting your computer from install/rescue floppy will then allow you to run fdisk normally on it and then rebooting normally. (I guess you can also launch your install from cd/floppy modify partitionning of the disk and then abort installation (without formating the existing partitions of course), and then reboot.

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