First of all, I know how to enable the Finder sidebar again, just by switching it on from the Finder preferences. However, from some strange reason, I've been experiencing that the sidebar has a tendency to disappear on some Finder windows, on at least two of my Macs (one running ML, the other SL). I'm not sure if the Mac running El Capitan suffers it or not. Since this has been happening since I upgraded one Mac to El Capitan, I'm thinking if it could be the cause, because I sync my files directory tree in all my Macs with rsync, so... I'm wondering if El Capitan changed any specification to the hidden "DS_STORE" files which might be causing this? One very noteworthy case is the root foolder of my USB drive (formatted with OS X format): One day its sidebar disappeared on my SL Mac. I enabled it again. I plugged the USB on the rest of my Macs (ML and El Capitan). And a couple of days later, I plug it again on my SL Mac and voila, the sidebar disappeared again. Note: I've never seen this in my Macs before, from Tiger up to Yosemite, and I've been syncing my data tree with rsync for years now. What's happening here?