Apple has released a couple updates for the iMac (late 2013).
This update is recommended for iMac (late 2013) with flash-only storage. This update improves the Power Nap feature in Mavericks on your flash based iMac by allowing it to perform silently, without spinning up the fan.
Power Nap periodically updates apps, such as Mail, Contacts, and Calendar, and will download software updates while your iMac is sleeping.
iMac 10.8.5 Supplemental Update 1.0 for machines running the NVIDIA GeForce GT 750M and OS X Mountain Lion v10.8.5:
Fixes an issue that may cause external drives to be ejected after the computer goes to sleep
Fixes an issue that may prevent certain USB Bluetooth adapters from working
If you own a late 2013 iMac and the above scenarios apply to you, then you will want to install these updates.