Mac crashes when watching videos. What's going on?

I'm not sure you'd want to use the backup from your other Mac, because to do so you'd have to erase that drive. If it's big enough, after erasing, you could create two partitions, one for each Mac.

Cloning a drive for the first time, however, can take some time and it might not make it through all the way. In that case, you'd have lost the data from the other Mac for nothing. So, this is kind of a rock and a hard place thing right now. 😟

If you've backed up your important stuff, then, if you determine the drive is failing, you can move that back over after you have a new drive installed. (I don't think your iMac is that easy as a DIY project. But depends on your skills.)

To get an idea what's involved, you could take a look at these excellent, illustrated instructions. You can find your model here. The EMC # is located under the foot of the mac.
http://www.ifixit.com/Guide/Browse/iMac

For cloning. I use Carbon Copy Cloner, freeware. It's very popular, easy to use and highly recommended. There are other ones out there, such as superduper, and tri-backup (this one not free.)

http://www.bombich.com/

Once you get this sorted out, you definitely don't want to be without an external, bootable clone.

SMCFan control
http://www.eidac.de/

EDIT: You could open up Console in Applications>Utilities. In System Log, see what kind of entries there are just before these events, and just after booting up again. There might be some clues there as to just what is going on. I'm not great at reading these, but you could post parts of these logs. Might be able to tell if it's heat related.

Message was edited by: WZZZ

Make sure you have the latest Adobe Flash Player.
http://www.versiontracker.com/dyn/moreinfo/macosx/10074404
Use the uninstaller to remove the older version:
http://kb2.adobe.com/cps/141/tn_14157.html

...only this time it made these really AWFUL repeated screeching noises.

Sounds from moving parts: this could be the hard drive or, less likely, one of the fans failing.
From the desktop Hard Drive Folder, in Applications>Utilities, open up Disc Utility. In the listing at the left, select/higlight the topmost icon. This is the drive. Down at the bottom, see if SMART Status shows "Verified" or not.

If it doesn't, this is a warning that the drive is about to fail completely. However, even if it reports "Verified," that doesn't mean the drive isn't about to fail. Drives can suddenly fail without any notice and SMART isn't that reliable for this. You should always have a backup, a clone of your internal drive, from which you can boot the computer, on an external drive. If you don't, get one ASAP and back up your internal drive and data before this disaster happens.

What iMac do you have?

I have an iMac 2006 edition also. About a year ago I started having "video issues" of all kinds while watching movies and gaming. My problem ended up being an overheated video card.

Cleaned out all the air vents with a vacuum made for electronics, blew canned air through as much of the case as you can and added a "fan control" utility that allows you to speed up all the internal fans manually. You can speed them up and leave them at higher speeds, but you will have a little more noise.

After doing this my video issues disappeared except for when I use a very intense game I have that pushes the video card to the max. This is when I manually speed up the fans and then I can game for hours.

Hope this helps.

Message was edited by: ECSD3R32

only this time it made these really AWFUL repeated screeching noises.

Common symptoms of a dying hard drive

Hard drive is making a clicking, whirring, grinding, ping pong ball noise or any other type of unusal noises.

Hard drive is not spinning.

Computer is not recognizing the hard drive.

Sluggish loading of applications.

System freezes and hangs.

An interesting site - Hard Drive Sounds by drive manufacturer.

Once you get this sorted out, you definitely don't want to be without an external, bootable clone.

But definitely don't try using a Western Digital external for this purpose. I've read in a number of places they can't be booted from.

These are good and reasonably priced.
http://eshop.macsales.com/shop/firewire/1394/USB/EliteAL/eSATAFW800_FW400USB