I have a imac Retina 4K, 21.5 inch, Late 2015 running Big Sur 11.6.5

Hi,

First, it appears that you do not have a Time Machine backup. If this is the case, you should create one. To start, you need an external hard drive. Here’s an excellent one sold by Apple: LaCie 2TB Rugged USB-C Portable Hard Drive - Apple. You will also need this cable for the external disk to work with your older iMac. You can then follow the steps in this Apple Support article to setup your Time Machine backup: Back up your Mac with Time Machine - Apple Support. 

Also, moving forward, I recommend that you use iCloud in addition to Time Machine to sync all your files and photos to all your Apple devices. Although iCloud is not a "true" backup service, iCloud can also act as an emergency off-site backup, but this is why iCloud should be used in addition to Time Machine. Here are some resources on setting up iCloud for Mac:

• Upgrade to iCloud+ - Apple Support
• Set up iCloud Drive - Apple Support
• Add your Desktop and Documents files to iCloud Drive - Apple Support
• Set up and use iCloud Photos - Apple Support

After you setup iCloud, turn off "Optimize Mac Storage" in System Preferences > Apple ID/iCloud. That way, all changes made to files in iCloud are also backed up to Time Machine. If you have this setting on due to a limited amount of storage on your Mac, that is fine. Just know changes won't be backed up to Time Machine, only iCloud.

However, reason your iMac is slow is due to the 5400-rpm hard drive. While this hard drive found in your Mac is perfectly suited to light tasks, such as email, web browsing, etc., for more advanced tasks, and for users who want the maximum performance from their Mac, this hard drive does have its limitations. Luckily, you can use an external SSD as your startup disk to run macOS and all your data from that. It will make your Mac 10 to 30 times faster for storage, and in turn, make your entire Mac system much faster. For more info, instructions, and what external SSD to buy, please see: How to Setup and Use an External SSD as your Startup Disk on a Desktop Mac - Apple Community.

Another option I would strongly consider is to replace this Mac with a new Apple Silicon Mac: Mac - Apple. You can also use Apple Trade-In to get credit towards a new Mac: Apple Trade In - Apple.

Hope this helps!

Jack

For starters the Late 2015 21.5" base model iMac's have a slow 5400 rpm hard drive.

If your 21.5" iMac has a spinning 5400 rpm hard drive, then there are several factors like free disk space, third party software and drive age that can make it even slower.

At this point I suggest getting and running EtreCheckPro on your iMac.

to get EtreCheckPro, go to > https://etrecheck.com

Then post back here with your EtreCheckPro report for examination and solutions.

as per > https://discussions.apple.com/docs/DOC-250000211

I'd replace it with a self powered drive from OWC (MacSales.com). Having their own power let them work without having to draw power from the iMac's port and, IMO, is more reliable. OWC is considered by many to be the premier 3rd party hardware provider for Macs.

Sounds as if the drive has failed. If you're going to replace it I strongly recommend OWC (MacSales.com). One of their OWC Mercury Elite Pro with USB 3.2 in the size (2 ½ 3 times the size of the drive being backed up) is an excellent choice as it's self powered and very reliable.

I have a number of their products and am a very satisfied customer.

Yes, you do need to erase and reformat the drive to APFS with GUID partition. The demo version of CCC will work. Be sure to Control (right) - click on the destination drive in CCC and select "Legacy Bootable Backup Assistant".

When I plug it in, it beeps several times, the light comes on, and I get this pop up message on the screen “The disk you attached was not readable by this computer. One of my options is to initialize, so I click that….it opens disk utility and it shows uninitialized.

Has it worked before, or has it always been in this state?

FirefighterDC wrote:

Yes, it has been working for several years and started acting up a couple of months ago.

Thanks for confirming. That, plus the beeps you hear, makes it sound like the disk is failing. I suggest you replace it with the LaCie I linked earlier.

Are you booted into the internal HDD drive? If so that's the problem. You can't erase a drive that you're booted into.

Boot into the Recovery volume (boot with the Command + R keys held down  - Intel Macs) or  (How to Boot an M1 Mac into Recovery Mode) and select Disk Utility. Then you can erase the internal HDD. Do not do this until you have your external SSD, format it APFS w/GUID partition and have cloned your boot drive to it with Carbon Copy Cloner.

Once you do that you can boot into the external SSD and erase the internal HDD without having to boot into the Recovery volume.

You will need to format the new SSD as APFS prior first. Use the Disk Utility app (Applications - Utilities - Disk Utility) and erase the new SSD and format as APFS, it will then be ready to be used with your Mac. Most external drives come pre-formatted for MS Windows boxes.

Regarding Carbon Copy Cloner, yes you should be able to use the evaluation copy to do your initial clone. However it may be wise to purchase the app and get another external HD and use Carbon Copy Cloner to create a daily bootable clone of your computer as a backup. If you already use Time Machine then it's simply wise to have a second backup because backups can fail too. I (and many others) do just that use both Time Machine and a Cloning app to make redundant backups. NEVER EVER use the same External Hard Disk for 2 backups though!!!!!!!!!!!

For a time, I booted my exact model of that iMac with an external SSD. I went from 93 MBps for the internal HDD to 450 MBps for the external SSD; a 5x increase in disk speeds. It was suddenly a joy to use.