Hi, system first attempts to load the image from the primary path. If boot fails, system will attempt to load the image from the secondary path (such as redundant), and finally, if that fails too, the system will load the default image. Unfortunately, I couldn't find a more descriptive document to share with you.    Hope this will helps!
I'm a software (Linux, virt, etc.) guy, not a networking guy. For my sins, I also manage my team's small lab, which uses a stack of Dell S3048-ON switches running FTOS 9. We'll be expanding the stack from 2 to 4 switches in a couple of weeks, and I'm taking the opportunity presented by having the new switches sitting around to research the details of how these things work and write some internal documentation. Right now I'm working on the OS9 upgrade process. I've reached the point that I've got the updated OS written to the B: partition of both switches. I'm pretty sure that the next step is to use the boot system command to set both switches to boot from their B: partition, i.e.: I've been unable to figure out the meaning of default/primary/secondary in this context. Having a default would make sense; having a primary and a secondary would make sense. The combination, however, doesn't seem to make any sense at all, and I've not been able to find anything that explains what these terms really mean. Anyone know?
Meaning of primary/secondary/default boot images? [S3048-ON OS9]