Q: Should I unplug my computer?

If you're not using a Journaling File System, data loss is very, very likely, since the operating system continues to frequently access the disk.  If you have a good Journaling File System, you're not likely to have problems the vast majority of the time, but it can still happen.  As suggested before, get an Uninterruptible Power Supply (UPS) for you iMac and keep your system running when the power goes out.  Just remember that the UPS is there to give you time to safely turn off your computer.  The UPS is not really meant for long term usage.  The UPS battery will need replacing sooner if you encounter many outages.

Generally, unless you're actively saving or copying files, you're not going to have data loss of your personal data.  The Operating System (OS) files, on the other hand could be in the process of saving data.  Most of that is log data and generic data information that are not critical to the actual operation of your system.  If you do lose data on critical OS files, you can easily recover it on the Mac by reinstalling just the operating system by booting up with Command r, then reinstall OS X.  This will not delete your Applications or Personal Data.  It will fully recover any critical OS files and your system should be working as new.

Duane wrote:

 

In the perfect world, perfect scenario, you are correct.

Don't want to belabor this, but this doesn't need a "perfect world, perfect scenario." Either the computer/disk is sleeping or not. If it is, then there is no drive activity, and a sudden, unexpected power loss can have zero effect on data, since no data is being written. Or if you think this is incorrect, then please explain why the need for a "perfect" scenario, which would seem to imply that the usual is an "imperfect" one.

As you just explained, that is the perfect scenario.

Perfect does not mean impossible.

Whatever might happen, it probably wouldn't be good. But I suggest that, instead of worrying about a bricked computer, and proposing these various catastrophic hypotheticals, you get a UPS* (I don't use one, but if you're that freaked out by the prospect of a sudden power outage, you should get one, and make sure it has a fresh battery) and have redundant backups on external hard drives, either Time Machine, or by way of clones (I use Carbon Copy Cloner, with two or more external drives. Some recommend using both TM and a clone.) That means two external drives, so the TM doesn't go on the same external as the clone. That way, if something goes wrong, which can happen in various ways without a power outage--drives can fail without any warning-- you can go back to the way it was before.

*Having redundant backups is far more important than having a UPS.

iOSAndroidRebel wrote:

 

But what if ... I am updating to MAC OS X 10.11.5 and it is in the black screen with an Apple Logo, and the power went off. Will it brick my Mac if I am still restoring my Mac to factory settings or update firmwares?

If the firmware is actually being updated and the power is removed there is a possibility of bricking your Mac. However firmware updates are very infrequent.

If the OS is being updated and the power is removed, there is a very high possibility that the OS will be corrupt and must be reinstalled. The Mac will not be bricked just unusable until the OS is reinstalled.

Since you seem intent on pursuing worst cases, see the following on restoring after an interrupted firmware update

About the Firmware Restoration CD (Intel-based Macs) - Apple Support