Hello Just like to say the 3850 can be in two kinds of boot mode - Install - Bundle (installpreferred ) and they are incompatible with each other - so if you try to attach a new switch to an exiting stack and the boot modes differ then it will fail to join unless you have the auto upgrade feature active on the stack otherwise you need to make sure of this boot mode prior to attaching the new switch and you can do that by show version If then the boot mode is different then you need to change it prior to it joining the new stack The procedure to change from bundle to install can be found on Cisco web site which will basically tell you to expand the bin file of the iOS and then change the boot system variable to boot from the conf file  I have limited access to the internet at present  to post the actual url link but I’m sure you should be able to find it. ... View more