Firmware patch notes says 5.0.9 officially supports upgrading from 5.0.6 or higher; try upgrading to 5.0.6 before going to 5.0.9. (But going on your past post, I am guessing you are trying this.)
If you are getting sig fault errors then it is possibly you may have a bad firmware image or corrupted boot disk or both. Perform a MD5 checksum on the downloaded image file against what is reported on the fortinet support site.
If the boot disk is corrupted, you will likely need to reformat it via the boot menu then TFTP the image over to the Fortigate. Entire process is covered in the Install and Sytem Admin
section of the FortiOS Handbook and else where
If you are still getting sig fault errors after formatting the boot disk then it looks like it' s time to open up a support or RMA ticket. Support will want you to run the HQIP
diag test on the Fortigate and record the entire console session via Putty or some other terminal program.
NSE4/FMG-VM64/FortiAnalyzer-VM/6.0 (FWF30E/FW92D/FGT200D/FGT101E/FGT81E)/ FAP220B/221C