Error 4MOT/4/400000003:HDD-1365

Backup IMMEDIATELY!!!!!!!!!!!!!!!!!!

Have the HD replaced by an Apple Store or Apple Authorized Service Provider! If you don't know any AASPs click the AASP Locator link.

A hard drive failure would be coded 4HDD; All 4MOT errors are cooling fan errors. Your error message is reporting a problem or outright failure of the hard drive's cooling fan. Not quite the expense but, as rkaufman says, BACK UP NOW! The lack of a cooling fan (4MOT) coudl cause the hard drive to fail.

The fan is buried at the bottom of the comuter case and replacing it is not an "at-home" task for even a reasonably adept home user. This picture from the iFixit web site shows the HDD cooling fan's position after almost everything else has been stripped from the computer:

Unless you are quite the computer disassembly expert, you'll need professional help to fix this one.

Slowdowns are more commonly due to software issues. Anti-virus apps, so-call "cleaning/optimizing/tune-up" programs, and some online banking security software slow more browsers that bad hard drives. So can incompatible browser plug-ins.

You can safely give us a snapshot of your config so we can see if you have any well-known bad software actors that can contribute to slowness. Please download and install this free utility:

http://www.etresoft.com/etrecheck

It is secure and written by one of our most valued members to allow users to show details of their computer's configuration in Apple Support Communities without revealing any sensitive personal data.

Run the program and click the "Copy report to clipboard" button when it displays the results. Then return here and paste the report into a response to your initial post. It can often show if any harmful files/programs are dragging down your performance.

Let's see what the Temps are. Get Temperature Monitor, free. http://www.bresink.eu/Downloads/TemperatureMonitor.dmg

If the drive temp is anywhere above 50 C. stop using it immediately. And even if it isn't, I wouldn't use it much longer, especially with anything CPU intensive. Get the fan replaced before continuing to use it, and I wouldn't worry about Etrecheck until that fan is replaced. At the same time as the fan is replaced, the drive can be checked out. The slowness might indicate that the drive is beginning to fail. But continuing to use it in this state will stress it unduly and dramatically shorten its life.

Please post back when you have resolved this problem. 99% of the times I've seen a HDD fan motor error the hard drive was failing. You didn't say which iMac you have and I'm not saying that yours is for sure but 2009-2011 ( I THINK) iMacs the fan motor is controlled by the hard drive.

Wouldn't that be the SMC, not the drive itself that controls the fan? I would think the strong correlation between a failing drive and a failing fan (if that exists--and I can see how that can be the case) would be that the drive was allowed to run long enough without adequate cooling, then causing irreversible damage.

Tthe circuitry for the temp sensor is built into the drive, thus the need to replace the hard drive with one from the same manufacturer or to install an in line temp sensor. Of course you could run fan control software. I *think* the SMC gets its information from the temp sensor and controls fan speed accordingly.

That doesn't sound right. The 4MOT error explicitly indicates a fan motor problem. It may be that the temp sensor on this drive is internal to the drive, unlike the pre-Late '09 predecessors, which used external stick-on sensors. But if this were a sensor error, it would show up as 4SNS something, not 4MOT, which specifically points to the fan motor.

What would have to be happening for your diagnosis to be even hypothetically correct, would be that the AHT is picking up the 4MOT error because of a drive temp sensor failure, causing the fan motor to stop working--which, for some reason, the AHT isn't detecting. In fact, what always happens (or at least should happen) when the SMC isn't getting any temp output from the drive (e.g., when someone tries replacing a drive in a Late '09 with an ordinary after-market drive lacking the internal sensor output to the jumpers--or, for the 2011s, lacking the additional leads on the power connector), is it ramps up the hard drive fan to its highest possible speed in order to protect the drive. Just the opposite of that hypothetical.

Hopefully the OP will report back when the problem is solved.