Can I back up to an SD card?
I use an SanDisk Ultra miniDrive (64 GB) for transferable storage. However, I use an external hard drive to maintain Time Machine backups.

Of note, for compatibility with Time Machine, the SD's default format (exFat) was incompatible, and I needed to use Disk Utility to reformat to Mac OS Extended (Journaled).

For for information about SD cards, Apple provides information.