Bootable SD card

[iTurbo]

Last night I installed Mac OS Catalina onto a 64 GB SDXC card. I did this for an emergency boot drive just in case the Fusion drive fails on my 2012 iMac.

At first I tried using the directions in 9to5Mac's YouTube video here:

That was only a very basic bootable volume with the installer on it which took up about 8.3 GB. Next I tried doing a full install of Catalina onto the card. This was very easy but you have to be sure to format it with APFS and GUID partition map. This also worked, but was unsurprisingly slow. Does anybody know how the internal SD card reader is connected to the iMac; bus/speed etc? System Report doesn't really say.

I am kind of surprised this worked at all. The speed issue isn't a huge deal, but I'm surprised that it seemed so much slower than the Fusion drive using it as a startup drive.
[automerge]1573919816[/automerge]

 

[redheeler]

SD cards were never designed to be used as a desktop boot drive, and most of them use cheap flash memory, so you aren't going to see very good performance.

 

[Fishrrman]

If you want a reliable and reasonably fast external boot drive (which is ALWAYS a good idea to have around), the best choice would be a cheap 120gb 2.5" SSD. You can find these for $20 now.

Put it into a $10 USB3 external enclosure or use a USB3/SATA adapter/dongle.

 

[sublunar]

I've not checked recently but I believe the SD card slot is connected via the USB bus, I don't think it's an exotic direct connection to a PCIe express lane or anything. Here's an Apple tech note on SD card slots.

The other issue with the SD card slot is only the iMac Pro support UHS-II for high performance cards.

It'd be interesting to see the transfer rate on a Thunderbolt 3 equipped device that could read UHS-II cards.

 

[kohlson]

I tried this a few years ago with an SD card I had lying around. Booted very slowly. More recently, I used BM Disk Speed Test to check a much faster (V30) SDXC. It came in at about 80MBps - about the speed of an HDD. There's more to boot speed that what's measured with this simple test, but you're way better off with @Fishrrman 's advice.

 

[iTurbo]

I found this article from Apple that answers a lot of questions regarding the built-in card reader.

Use the SD and SDXC card slot on your Mac - Apple Support

Learn how to use the SDXC slot on your Mac.

support.apple.com

Apparently laptops use the USB 2 bus, but the desktops use the PCIe interface. My 2012 iMac reports the link speed as 2.5 GT/s. I'm not really sure how that compares to something like SATA.

Anyways, it was kind of fun to experiment with it. It is quite slow when using as a boot drive. It would be neat to do some benchmarks with it to see how it compares to other storage methods.

 

[sublunar]

I found this article from Apple that answers a lot of questions regarding the built-in card reader.

Use the SD and SDXC card slot on your Mac - Apple Support

Learn how to use the SDXC slot on your Mac.

support.apple.com

Apparently laptops use the USB 2 bus, but the desktops use the PCIe interface. My 2012 iMac reports the link speed as 2.5 GT/s. I'm not really sure how that compares to something like SATA.

Anyways, it was kind of fun to experiment with it. It is quite slow when using as a boot drive. It would be neat to do some benchmarks with it to see how it compares to other storage methods.

Sounds like a single thunderbolt 1 pcie lane on the 2012. Don’t forget that the port itself may not support high speed cards. Flash cards won’t have anywhere near the wear resistance and read write flexibility of SSD though.

 

[priitv8]

It'd be interesting to see the transfer rate on a Thunderbolt 3 equipped device that could read UHS-II cards.

Here you go!
CalDigit TS3 Plus dock and Sony SF-M64 R260/W100 card.