About a month ago, I had to upgrade my late 2009 27" iMac from Mavericks to El Capitan due to some apps that only worked in El Cap. A major problem for me in El Capitan, is that now my computer has great difficulty recognizing external hard drives, specifically two drives that are both RAID1. Both have 2TB Hitachi Hard Drives in them (so altogether 4 HDDs), and are housed in two separate enclosures. The RAID1 on both is a hardware RAID implementation. I use them to store music on, controlled through iTunes, and I play music from them. Under Mavericks (and earlier under Mountain Lion, and even earlier under Snow Leopard) I had the drives connected to my iMac at all times, and there was no problem recognizing them, ever, including when I would reboot my iMac for whatever reason. Now under El Capitan I'm having constant problems. If I re-boot my iMac, often the drives won't come back on. I have to disconnect the FW800 and re-connect, or turn off the drives then turn them back on, several times. Also, sometimes what happens is that out of the blue I'll get the message "disc improperly disconnected" message and the drive disappears from my desktop. Finally, a couple of days ago, I got that situation again - "disc improperly disconnected" - even though I didn't touch the drive in any way, nor was I using it at the time, it was just sitting there connected as always. Only this time it has not come back on, no matter what I do - connect, disconnect, hook up with FW800, hook up with USB 2.0, turn the drive on, off, re-boot the iMac with the drive connected and without connected and only connect later. I've even reset SMC. I've tried going through a USB or FireWire hubs, and also going directly to the iMac, trying various USB connectors, changing USB and FW800 cables. Nothing. Meanwhile on the RAID enclosure I have various LED lights that tell me what is happening to the drives. It shows that the drives are healthy, and not being accessed - so the iMac is not connected to the drives, seemingly. Nothing shows up in Disc Utility. I'm out of ideas. My final idea is to just try and upgrade to macOS Sierra in hopes of resolving this. I would downgrade to Mavericks, but unfortunately I can't due to apps I need. So my question: are you guys having any problems with macOS Sierra and external RAIDed drives? Thanks!