My Mac is doing strange things: delaying between users, mouse flickering and bouncing, gets stuck with rainbow wheel flickering. I ran the appel hardware test and it detected an error: 4MOT/4/40000003:HDD-1233 Does anybody know what that means? HELP!

I seem to remember this from awhile back, that MOT means motor as in fan motor, and HDD is hard drive. I think it means the logic board called for a fan(s) to come on to cool the hard drive but somehow it didn't happen, either b/c the HDD thermal sensor was bad or something on the logic board not working. Do you hear excessive fan noise? Or no noise? Do you have iStat Pro, which monitors fan activity?

Be sure you make a backup if you don't have one. Do you have AppleCare? Which iMac and hard drive do you have (in System Profile)?

You should quickly backup any important files just in case, then run the Disk Utility application located in the applications/utilities folder. Use the 'verify disk' option and see if it reports any major errors. Depending on the status of the hard drive you can attempt to 'repair' the hard drive using the 'Repair Disk' option, however you may need a new hard drive.

Good luck.

iStat Pro will display temps and fan speeds. If a fan isn't working, it will show up running very slowly or at zero rpms.

http://www.islayer.com/apps/istatpro/

It can either be the fan or the drive.

Run the free demo of SMART Utility to check the drive.

http://www.volitans-software.com/smart_utility.php

WZZZ answered about where to get iStat. And do check the SMART status. If it is an overheating problem due to a fan or logic board problem, your hard drive is possibly cooking itself to death. If so it isn't a faulty hard drive even though the hard drive might fail. So assuming it's a temperature problem, even if you are able to repair things on the disk with software, that is working on symptoms, not causes. I could be wrong however.

RE: AppleCare: Your iMac came with one year of AppleCare (Apple's warranty program), but within the first year you can buy 2 more years. You have to extend by the one year anniversary of purchase of the computer. Your 10,1 is too old to still be in the first year, and since you asked what it was, I'm sure you don't have it. Bottom line meaning is that whatever this problem turns out to be, you'll have to pay for it. Unless there is something like this. It is for 2011 iMacs with certain Seagate drives. You can put in your serial number for fun, but it looks like yours is too old. Lastly, some people have had Apple help them anyway if it is just out of warranty, but many have not. Your machine is one of these. Type in 10,1 in the search box. Is there an Apple Store near you? Just b/c it's out of warranty doesn't mean you shouldn't have it looked at by Apple. But no one here can say at all what Apple will or will not do.

Hope you get it taken care of!