Hello. I have an IBM x3550 server with five 500GB SAS 7.2K slim-HS drives, configured as RAID 5. Currently the OS is not installed. Can I add one more similar disk to the array AFTER installing Debian OS?
Look at the RAID GUI, to see is there a RAID expand option.
Since you're using SAS controller (SAS drives), most likely the option is there.
You can expand the RAID AFTER installing OS, if the option is there,