Hash Mismatch prompt displaying AGAIN after (I think) a complete disk wipe and re-install of Ventura 13.1.

Please run the third party app Etrecheck and post the report here so we can examine it for clues.

Also try running the Apple Diagnostics to see if any hardware issues are detected.

For many of "Volume Hash Mismatch" errors, they are being caused by bad memory. Unfortunately the Apple Diagnostics rarely find hardware issues. You can try creating & booting a Memtest86 USB stick to run a more thorough memory test, but even this third party memory test does not always find bad memory.

Here is a link to another post where a contributor has linked to other similar Apple forum posts about this issue:

macbook hash mismatch detected volume dis… - Apple Community

I have seen multiple people confirm on this forum that Apple replaced their laptop's Logic Boards with some definitely failing a memory test on the Apple service diagnostic, but I don't know if those links reflect any of those confirmed bad memory being the source of the problem.

Just to further validate @HWTech statement regarding possible Bad RAM Modules

The report is showing " 72 GB RAM - Upgradeable " which does not appear to be a Normal Configuration for RAM

Apple Computers are extremely picky on RAM used on their computers

If there have been some After Market RAM modules installed - perhaps removing some of the After Market RAM Modules and testing of the " Hash Mismatch " issue is resolved

Only for comparison form other users experiencing the same type of " Hash Mismatch " issue

Volume hash mismatch, cannot reinstall MacOS

Monterey rev 12.4 - Hash mismatch

Please help - Terminal.app keeps crashing upon reopening - macoS Monterey version 12.4

Volume Hash mismatch

Strange Disk Space Issue

RAM Failure 

reichc4075 wrote:

I do have aftermarket RAM. I've been getting away with using it since I purchased the machine - about 3 years. I'll pop it out and put in the RAM that came with the machine.

I'm also attaching photos of Disk Utility.

You may try again using the Image Insertion icon

It is the Second to Last icon in your reply posting.

The 2nd image (named Disk Utility 2) indicates the partition isn't mounted, but it's showing active memory usage in images 3 and 4 (named Disk Utility 3 and named Disk Utility 4).

Is this suspect?

I agree with @PRP_53 that your memory modules are most likely the problem. One could be bad, or if they are third party upgrades, then they may not be compatible with the iMac since Macs are very picky about the memory they use so we usually only recommend using memory from Crucial or OWC. Even then, make sure to only buy the exact part numbers suggested by using the tools on their respective websites for identifying compatible memory for your specific iMac. FYI, it is extremely rare to have more than one memory module fail at the same time.

If the file system has been corrupted, which can happen when there is memory problem, then usually the only way to fix the file system when First Aid is unable to repair it, is to erase the disk followed by reinstalling macOS & restoring from a backup. There is a slim possibility the memory problem is interfering with some calculation of those hashes when running First Aid, so fixing the memory issue could possibly solve the Volume Hash Mismatch....very hard to say though since I've never personally encountered this issue.

Unfortunately the EtreCheck report is not showing the exact memory configuration, but it would seem you have mixed the original Apple memory with a third party upgrade. I would try using the Apple Diagnostics and Memtest86 to see if you get any memory failures (including a system freeze during testing) noted in the current configuration. Having a repeatable failure in the current configuration makes it much easier to identify the problem memory since you can remove one module at a time to see if the tests pass (only works well if the failure is repeatable). I would start with the new memory since that is more likely the source of the problem in my own personal experiencing supporting thousands of my organization's Macs.

I must say, when my " Go To " Contributor, @HWTech, offers advice related to and especially hardware issues.

I personally pay special attention to the ideas put forth.

reichc4075 wrote:

I do have aftermarket RAM. I've been getting away with using it since I purchased the machine - about 3 years. I'll pop it out and put in the RAM that came with the machine.

Was that memory all purchased as a single kit?

Or several kits of paired memory?

I know some PCs can be very picky about memory if it is not paired with memory from the same manufacturing batch. Crucial techs mentioned this to me years ago which is why Crucial sells memory kits to minimize having issues like that occur.

I'm also attaching photos of Disk Utility. The 2nd image (named Disk Utility 2) indicates the partition isn't mounted, but it's showing active memory usage in images 3 and 4 (named Disk Utility 3 and named Disk Utility 4).

No, that is all normal. Their are two read-only signed & sealed APFS volumes reserved for the core macOS system files. One is used for staging new macOS updates while the other one is used when booted into macOS.

About the read-only system volume in macOS Catalina or later - Apple Support

Signed system volume security in iOS, iPadOS, and macOS - Apple Support

Role of Apple File System - Apple Support

Unfortunately Apple tends to skip over some parts including the double copy of the system volume. FYI, the second copy used to be named "Update", but Apple took the name away probably because Apple toggles between the two volumes now so the role of those two volumes changes.

Welcome

reichc4075 wrote:

I downloaded Memtest 86 but I actually don't have a usb flash drive. Can I use an SD card instead? Or an external hard drive? If I need to find a USB Flash Drive, I bet I can, but this is what I have to work with right now.

I believe you can use an SD card, but I've never tried it.

Not sure what you mean by "flash drive" since that has several meanings (SSD or USB stick/thumb drive), but a USB stick or "thumb drive" is usually used since they are smaller in size and used for temporary storage, but just about any USB based drive can be used (depending how the Memtest86 drive is created, it may destroy all data on the destination USB drive and only use less than 10MB of space).

So far, things feel much better on the machine and it is more fluid with less jerkiness. But I will miss the loads of RAM I had before.

If you were using OWC RAM, then most likely just one of those modules is bad. The trick will be in identifying which one is bad. It is very rare to have more than one memory module fail at the same time. Probably best to test the original Apple memory first just to make sure Memtet86 runs fine with it. Then remove the Apple memory and take two of the OWC modules to install & test. If no errors & no system freezes while running Memtest86, then rinse & repeat with the next two OWC modules.

If you receive an error or system freeze when running your first test of the OWC RAM, then pull one of the modules making sure to keep it separate from the other memory modules. Test the single module to see if there are any issues. If no issues, then swap this module with the one you set aside and repeat. Make sure to keep both of these modules separate from all the others for now especially if you don't encounter any issues when testing the 2nd module by itself.

If you find an issue with a particular single module, then place it to the side making sure you don't mix it up with the others.

If Memtest86 has no issues with most of the OWC RAM, then those that had issues are likely bad. I think OWC will respect the results of the Memtest86 results especially if it only had issues with one or two modules.

Would you have any suggestions on how to proceed without a USB Flash Drive? Or is it time I just go get one?

If an SD Card doesn't work, then you can use any USB drive (SSD, hard drive, USB stick/thumb drive) available, but depending on how you create the bootable Memtest86 USB drive....it may destroy all data on that drive.

FYI, Sometimes you may find Disk Utility may have problems erasing a drive after using it to make a bootable USB drive from an .iso or .img file, but it can be easily resolved. macOS & Disk Utility are not very smart and are easily confused. Just let me know if this happens and I can provide instructions to resolve it.