I cannot expand RAID5 capacity on an Adaptec RAID 71605, an "expand" in ribbon is greyed out

In order to do an expand, Adaptec recommends to use the latest Firmware on the RAID controller, check that consistency check is disabled and to initialize the disks you want to add to your RAID array. You can find in this document more details about this process.

Always have an updated full backup of your information, independently the size of it. This is the most basic action you can do to protect it.

If there are partitions on the HDD, it cannot be initialized or added to an array. The entire HDD must be unallocated.

If this is your situation, remove any partitions and try again.