Can I use the Recovery Partition on one startup disk to repair another (internal) disk? I have an SSD and an HDD in my Macbook Pro (Yosemite) and I recently cloned the HDD to the (new) SSD.  The SSD is now unhealthy, for reasons unknown and Disk Utility recommends repair from the Recovery Partition.  Unfortunately there is no RP on the SSD, but it happens that the old HDD still has one, as I have not formatted it yet.

Can I run the RP on the HDD to fix the SSD?  If I have this other startup disk, do I even need to start up from the RP?  Can I just do this from the Disk Utility when starting up from the HDD normally?
You may use either the Disk Utility on your Recovery partition or the one on the main volume of your HDD to repair the SSD as long as the Disk Utility version is not too old. It's recommended to use a similar version of Disk Utility as the one installed on the volume to repair.