iMac 5K

15
Dec
2025

The Crucial BX500 SSD was a mistake in my case

A couple of weeks ago when I replaced the blown Crucial MX500 SSD system drive in my iMac 5K 2019, I made what I now think was a mistake in choosing the Crucial BX500. I had read that the MX500 that I had been using for years was end of life, and I assumed the cheaper BX500 might be its replacement. But far from it.

The warning signs were that in recovering the system, it took an exceptionally long time, and frequently displayed the prohibited icon. This, despite knowing the correct system was installed on the drive.

Prohibited icon

I consulted Claude.ai in search of insight beyond the advice in the Apple support article. It asked the model of the drive being used:

Claude: What type of SSD is installed?

Me: it is a crucial bx500 ssd aftermarket, replacing a prior aftermarket ssd that had failed following a problem with the power supply board

Claude: That explains a lot. The Crucial BX500 is a budget SATA SSD that’s known to have compatibility issues with Macs, particularly with the boot process. Combined with your iMac’s power supply history, there could be multiple factors at play.

Claude: The BX500 issues with Macs:

  • Often has NVRAM/boot detection problems
  • The prohibited symbol cycle you’re seeing is a common complaint
  • Eventually boots but takes multiple attempts
  • Not officially Mac-compatible

Since at the time the only symptom I was aware of was the prohibited icon appearing on a cold boot, and not really wanting to spend more on this repair, I hoped I’d be ok only sleeping the system as a rule. After all, the restore had successfully written around 800Gb of data.

Unfortunately over the next weeks, more of the reported symptoms started to appear. Spinning beachballs, system slow-downs etc. These seem to happen when lots of small files are being written especially by browser activity and time machine back-up activity. One two occasions the system eventually crashed. Prior to using this drive it never crashed.

Claude suggested, after I told it MX500 was being phased out:

  • Samsung 870 EVO (widely recommended for Macs)
  • OWC Mercury Extreme Pro (Mac-specific)
  • Western Digital Blue SA510

I asked Claude for a comparison between the BX500 and the WD Blue 510

*** Claude start ***

Technology & Architecture:

  • BX500: Uses QLC (quad-level cell) NAND technology Best Buy, which stores 4 bits per cell, and lacks DRAM cache (uses Silicon Motion SM2258XT DRAMless controller) SSD TesterTechPowerUp
  • SA510: Uses Western Digital’s own 3D TLC NAND technology B&H Photo Video (3 bits per cell) with SLC caching that temporarily boosts write speeds B&H Photo Video

Performance:

Both drives advertise similar peak speeds:

However, sustained write performance differs dramatically:

  • BX500: After writing 25 GB, write speeds drop to around 100 MB/s TechPowerUp. In the 2TB model, after approximately 184 GB, write performance is throttled with average write speeds of 151 MB/s Best Buy. After filling 60% of volume, speeds can decrease to a very low 60 MB/s Western Digital
  • SA510: After approximately 54 GB, the pSLC-Cache was full with average write speed of 287 MB/s Product Chart – significantly better than the BX500’s post-cache performance

Reliability & Endurance:

  • BX500: Among the lowest endurance ratings on the market, starting at 40 TBW and spanning up to 120 TBW SSD Tester
  • SA510: 400 TBW endurance rating with a five-year warranty B&H Photo Video – over 3x better than BX500

Mac Compatibility:

  • BX500: Known issues with Macs, including write speeds hovering around 10-15 MB/s on iMacs even after fresh OS installations Tech Atlantix
  • SA510: Better general compatibility, fewer reported Mac-specific issues

Bottom Line

The WD Blue SA510 is the superior drive with better sustained write performance (287 MB/s vs 100-150 MB/s post-cache), much higher endurance (400 TBW vs 120 TBW), and better Mac compatibility. The BX500’s QLC NAND and DRAMless design make it struggle with sustained writes – exactly what you’re experiencing with your migration and speed tests.

For a 2TB upgrade in your 2019 iMac, the SA510 would be a significant improvement over your BX500.

*** Claude end ***

WD Blue SA510 SATA SSD 2TB

I was persuaded, and got hold of the WD Blue SA510. First using an external USB 3 enclosure, I repeated the system recovery and time machine recovery to that drive – taking at least 2 hours less time, and not seeing the prohibited icon once. The Black Magic Disk Speed Test showed consistent read and write of around 350Mb/second. With a few days operation and no slow-downs, spinning beach balls, or prohibited icons on a cold boot, I moved the drive to the internal SATA without issue, and there seeing writing consistently at 450Mb/s and read 483Mb/s. This time I have the confidence to re-stick the screen.

I do have a BX500 1TB in my PS4, and it has been working perfectly well there without any noticeable issue. Maybe this one will be ok as an archive drive – but not for anything requiring lots of sustained writes.

Posted by creacog
28
Nov
2025

iMac 5K 2019 repair – Blown power supply board

TLDR

When I upgraded the fusion drive to SSD, I opted to include the OWC in-line Digital Thermal Sensor. That sensor was the root cause of the repair needed here, and ironically, I don’t think it was ever needed in this model iMac.

Detail

A couple of years ago, I removed the 1Tb fusion HDD from my iMac 2019, to replace with a 2Tb Crucial MX500 SSD. There was some confusion at the time as to whether the iMac would depend on the drive having a thermal sensor, without which it may run the fan at full speed always. Therefore I got hold of the OWC bundle including Digital Thermal Sensor and Glue strips and installed as written up here.

After just over 2 years of faultless operation there was a loud ‘crack’ noise from behind the screen and the iMac was instantly dead – with no means of powering it up. I was pretty convinced that the power supply board was the affected component and ordered a replacement (2nd hand) via ebay, and avoided opening up the iMac until that had arrived.

I then started work cutting the screen glue with the old iFixit cutter. It felt much more difficult than the original glue a couple of years ago, and at some point I managed to break the cutter wheel.

iFixIt cutting wheel - broken blade
iFixIt cutting wheel – broken blade

At which point I ordered a new cutter with spare blades via Amazon. I then made a crutial mistake… I attempted to continue working around the screen but using a pick tool. I should have just been patient and waited for the next day delivery, as the pick tool caused extra stress on the screen resulting in a chip of glass breaking free and some hairline cracks. Fortunately the worst of it is in the black bevel are, and I have simply covered with black electrical tape.

With the replacement blades the next day, I was able to remove the screen. At this point the root cause of the problem was immediately apparent. As you can see from the photo of my original work, the OWC sensor is stuck to the SSD.

Replacement drive in its mounting kit with its inline temperature sensor

With the screen removed, we can see that the sensor is nolonger stuck to the SSD, and has dropped, with the wires just long enough for the sensor to land on the power supply board. Closer inspection shows soot on the sensor board and missing components, and against one of the transformers of the power supply. The back of the sensor had lost all stick – the glue has completely denatured.

Inside iMac 5K 2019 showing the OWC inline thermal sensor unstuck from the SSD.
Close up of the OWC in-line digital thermal sensor, showing scorch marks and damaged components
Damaged OWC in-line digital thermal sensor
Close up of the visual damage to the power supply board.
Damaged power supply board, close up

I did have some worry about the power supply board that I had obtained via eBay – it was from an iMac 5K 2020, and while it looked the same size, its components were in a different layout. It does turn out that the board works well in this iMac.

The different component layouts of power supplies.

However, once installed, it was apparent that the SSD was no-longer functioning. So the next step, I ordered a Crucial 2Tb BX500. This turns out to be a potential mistake. On a cold boot, the Mac shows the prohibited icon for a while. This eventually clears, but the drive performance, particularly in the time machine restore is very poor.

New BX500 SSD in place, without any thermal sensor this time
New BX500 SSD in place, without any thermal sensor this time

In consultation with Claude.ai, the suggestion is that the BX500 is a budget drive and has some Mac compatibility issues. However following a few weeks basic operation which only involves creating and editing a few lightweight documents and online streaming, I have not hit any noticable issues – other than on a cold boot and that ‘prohibited’ icon.

The fan has been running normally, therefore I do not believe the thermal sensor was actually necessary in the first place, and my belt and braces approach has turned out to increase rather than decrease risk on this occasion.

If you decide to install the sensor (or already have) i recommend using cable ties and routing the sensor wires such that when it comes unstuck, it cannot fall into the power supply, or touch any other metal component.

Posted by creacog, 0 comments