"- what is best scenario to replace the 2 HDD" If you have two drives that need replacing in a RAID 5, there is NO way to replace them while keeping your data intact, because your data is gone.  You replace them, configure a new RAID 5, then restore from backup. "- can use HDD from another manufacturers " but same size and speed" to rebuild the Raid" You don't need to use disks of the same size - they can be larger.  You should use Dell-certified drives.  You should NOT use consumer/desktop drives in a server. If you have one disk showing as foreign, then you may be able to get your array back online.  Go to CTRL-R, highlight the controller on the VD MGMT screen, F2, Foreign Config, Import.  If it imports and everything works again, then you can replace the remaining disk by simply pulling it from the system and inserting a new one.  If it does not begin to rebuild automatically, then you will need to assign the disk as a hot-spare to begin the rebuild. 
Dears i have Power edge T620 with PERC H700 and  4 HDD 500 GB Raid 5 , and one give me missing , and another Foreign , the server host ESXi 4.1 and ESXI up and run but each time i tray to start one of VM's system crash , now i have some Q : - what is best scenario to replace the 2 HDD  - can use HDD from another manufacturers " but same size and speed" to rebuild the Raid   THX
Hared disk replacement