A sure way is to remove the disk from the NAS, find out the make/manufacture. Bung it in a desktop and run the manufacturers diagnostics. These run from a boot floppy and can be downloaded from their website.

If the disk passes with no errors then a low level format would be a good idea, again can be run from the floppy created by the diag prog.

Once that is done, go back into array manger and take it from there, break the mirror first, then upgrade the newly formatted disk to dynamic. Right click on the original disk and select add mirror, select the disk and it will create the 5gb os partion.

When complete, right click on the raid 5 array and select re-activate. Then the rebuild option should become availabe. Let that rebuild, once all is finished, then you can delete the missing disk entry.

A good guide is here: http://support.dell.com/support/edocs/stor%2Dsys/spv715n/en/admin/index.htm

Alternatively, web into the nas and log into NAS Manager, in the administrative tools, select disk management, on the next screen click on volumes. The next screen should have an entry to "repair volumes". Click on this and it will rebuild everything with no further intevention. if this is not available then you'll have to use array manager.