IMac desk top - prohibitory symbol, I ran first aid main disk issue

If you are referring to your startup drive, please restart in the Recovery Partition (hold down Command + R on startup) and open Disk Utilities there and execute First Aide. HOWEVER, your start up drive is likely failing and you either need to replace it, replace the computer or seek another alternative such as using an external SSD as your startup drive.

As you did not provide any specifics about which iMac you own we cannot make any suggestions for a direction for you to take. If you want to post back with that info (please be specific such as "2015 27" iMac with 7200 RPM hard drive and 8 GB RAM") so we can help.

Craig,

My recommendation is that you take the computer to your local independent Apple Authorized Service Provider to have the computer professionally diagnosed. If you do know of an independent Apple Authorized Service Provider please click Apple Authorized Service Provider Locator (AASP) & Genius Appointment and then click the Find an Apple Authorized Service Provider link. Then you can get a professional assessment. They will be able to tell you which model year iMac you own, if it is older than a 2017, I strongly recommend replacing it rather than spending money on it.

As you don't seem to have backed up the computer this means your data and apps will be gone. This will be lesson that operating ANY computer without a well thought out backup plan is a guarantee of lost data. Apple provide Time Machine with EVERY Mac, all the user has to do is provide an External Hard Disk such as a OWC Mercury Elite Pro to back up to. Many experienced users buy 2 EHDs, why to have redundant backups to ensure that if one backup fails they have another. Don't laugh, this happens every single day. The 2nd External Hard Disk is used to create a bootable clone using an app such as Carbon Copy Cloner or SuperDuper! After all, what value do you put on your photos, music, financial information, etc?

Any Mac with 8GB or more of RAM and the largest SSD you can afford, the minimum I would buy is a 512GB SSD however if your old Mac had a 1TB HD it would be wise on the new Mac to buy one with a 1TB SSD. Also purchase one with an M1 microprocessor. Apple has moved away from using Intel microprocessors and are using their own so move forward rather than trying to live in the past!!!

Prior to buying your new machine ensure all of your third party apps are 64 bit apps and will run on current versions of Mac OS which currently is Monterey. Once you have gotten the old machine in shape, create a current Time Machine backup and then when the new machine arrives follow the simple steps in Setup Assistant and Migration Assistant and your migration and experience will be great.

You are not following directions. The instructions I provided will tell you the model. year iMac you own, please check About this Mac again and you will find this information! However, on a 21.5" iMac that has a failing HD I would recommend that:

1. Backup your computer NOW using Time Machine, your HD is failing and you need to protect your data!!!
2. I recommend that you replace the computer with a new 24" iMac or an Apple Refurbished 24" iMac with the largest capacity SSD you can afford. Another option is to buy a new (or Apple refurbished) Mac mini with the largest SSD you can afford.
3. Buy a USB 3 external SSD such as OWC Mercury On The Go Pro and a OWC On the Pro Go Power Adapter and use a cloning app such as SuperDuper! or Carbon Copy Cloner to clone the internal HD to the new SSD. Then use the external SSD as your startup drive.

To learn which iMac you own, please click the in the upper left of the display and the click About this Mac and it will provide you with the model year of the iMac and how much RAM is installed. Please provide that information.

Also, did you start up in the Recovery Partition as suggested?????????????