Booting from an SD memory card? SDHC, SDXC 32GB or 64 GB... beware!
I have a SanDisk 32GB SD card (bought at Costco about 6 months ago to use with a camera for making home movies) that is rated at
80MB/sec. My 7200 RPM hard drive is rated at
480MB/sec but never delivers anything over
50MB/sec when connected externally via USB.
The SanDisk hi-speed SDHC will boot
Yosemite 10.10.1 in about
3 minutes. The 7200 RPM hard drive (external USB) takes
2 minutes to boot the same environment. The SD card is plugged into the slot on the side of my MacBook Pro (2009 vintage.) The USB Hard Drive is plugged into the USB socket on the side of the MacBook Pro. The same 7200 RPM USB hard drive will boot
Snow Leopard 10.6.8 in under a minute as an external boot drive.
I got an off-brand "high speed"
SDXC 64 GB card, and it took more than
5 minutes to boot the SAME cloned 20 GB Yosemite partition.
EDIT: Flash memory technology is a very different animal from SSD memory technology. The electrons spin in different directions (j/k LOL) but yes, the performance of SD card vs. SSD will always be a disadvantage. For a temporary, spare, extra boot environment SOME SD memory cards may be okay, some not. Be prepared to take a few hours to test it and you may need to take it back if it is too slow.
The 64 GB SDXC took about SIX hours to fully ERASE. The 32 GB SDHC took less than ONE hour to fully erase.
If you want good SD memory card performance for a spare boot up environment be sure your SD memory card is rated at
80MB/s or better, and even then you may be disappointed, depending on the manufacturer and the firmware and performance of the specific memory chip. One mfgr. rated at 50MB/s does not even come close to half the performance of another mfgr. rated at 80 MB/s. YMMV.