Note: I am new to this forum. If this type of question is not appropriate for where it is posted, please accept my apologies and let me know where to go if you can. Thanks. Yesterday I decided to upgrade the hard drive in my 2011 21.5-inch iMac according to this guide. Since booting up after installing the new drive, my system is not recognising that there is a drive installed at all. Disk Utility doesn't show it, and it doesn't even appear in my /dev directory. I can still use the machine, as the OEM hard drive was/is not the drive on which OS X is installed. About a year ago, I installed an SSD according to this guide, and have used the OEM disk as storage. The only other hardware that has been changed is a RAM upgrade I installed shortly after purchasing the machine in 2011. The new hard drive is a WD Blue 4TB, which replaced the original WD Blue 500GB. It may also be worth noting that the hard drive is not even recognised when booting from an OS X recovery USB drive. I appreciate any help or insight, and if this particular problem has been solved in another thread, then I kindly ask that you point me in the right direction. Detailed specs for my machine are as follows: OS X 10.11.2 iMac (21.5-inch, Mid 2011) Processor: 2.5 GHz Intel Core i5 Memory: 12 GB 1333 MHz DDR3 Startup Disk: SSD 224G Graphics: AMD Radeon HD 6750M 512 MB Edit: apparently one cannot use \[CODE\] formatting inline.