I think the issue with SD cards is that is not considered a "Local" (hard) drive.
Note: Scratch space is configured automatically during installation or first boot of an ESXi host, and is not required to be manually configured. ESXi Installable creates a 4 GB Fat16 partition on the target device during installation if there is sufficient space, and if the device is considered Local.
You may have to refer to the KB # Creating a persistent scratch location for ESXi 4.x/5.x/6.x (1033696) | VMware KB
Below are the comments to your query same has been mentioned on the KB .
These are two examples where scratch space may not be automatically defined on persistent storage. In each case, the temporary scratch location is configured on a ramdisk:
1. ESXi deployed on a Flash or SD device, including a USB key. Scratch partitions are not created on Flash or SD storage devices even if connected during install, due to the potentially limited read/write cycles available.
2. ESXi deployed in a Boot from SAN configuration or to a SAS device. A Boot from SAN or SAS LUN is considered Remote, and could potentially be shared among multiple ESXi hosts. Remote devices are not used for scratch to avoid collisions between multiple ESXi hosts.RAJESH RADHAKRISHNAN
VCA -DCV/WM/Cloud,VCP 5 - DCV/DT/CLOUD, ,VCP6-DCV, EMCISA,EMCSA,MCTS,MCPS,BCFA
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I'd seen the information that the scratch partition isn't created on flash memory, but I wasn't sure if that was still a hard rule and if there was a supported configuration that would allow it to be created on an SD card. With the improvements in the flash memory, I thought it might be a valid option.
Part of what was confusing me was the info in KB article 2004784. From the text of the article, it seems to say that if you have an SD flash card 8G or over, it WILL create the scratch partition. That article showed changes to boot.cfg to allow it. It was referencing ESXi 4 and 5, but I wasn't sure if that was supported on version 6.
I recently switched our hosts from boot from SAN to boot from SD card. We had a host PSOD ..... no logs. Vmware confirmed as of last week that a scratch partition is NOT automatically created on an SD card. You would think this would come up as a big flashing option during the install. Something that says "Hey you !!! Do you want to create persistent storage for your logs?"
I can understand when a "Large" SD card was 8GB, you might not want to automatically create a 2GB scratch partition. But now?
So my question is... is there an easy way to create and use a partition on an SD card?
If this is the article you are meaning (https://kb.vmware.com/s/article/2004784) then the versions of ESXi it applies to include ESXi 6.x so by implication the change must be supported
I haven’t had a chance to test the boot.cfg changes as listed but the implication is strong that this will force create a scratch partition on a USB/SD device.
For me I would still be wary of this as the write cycles are significantly lowly with these devices than other storage. While there are improvements with quality for SD/USB devices, VMware cannot guarante what device you use and don’t certify these. Take a look how ESXi changes the writing of configuration changes to prevent excessive writes to this type storage, e.g. hostd is limited to 6 backups in an hour.
ESXi 6.0 U3 even introduced an advanced parameter to offload the VMware Tools ISO to RAM in order to reduce read wear on SD/USB devices!