Why does new Time Machine & Migrate fail with SD

Question

My MacBook Air had a Time Machine backup of my 60g partition on 80g partition of SD card. The drive needed reformatting, but when I tried to restore the TM backup after disk was erased, CMD-R wouldn't boot the SD card--it would see it but not boot.

Now, I've destroyed that TM backup and completely started over both with a new encrypted 80g system partition (OS X 10.11.5 El Cap) on the Air, and 95g Time Machine partition (exfat) on the SD.

Last night I did a complete TM backup. But this AM when I attempted to test a restore via Migration and CMD-R at startup both attempts failed.

Migration app could not recognize the SD, CMD-R would see the SD volume but when I'm at the Select System Backup screen, the file browser window shows the message, No system backups were found."

This seems to be more complicated than I expected. Should I be doing more reading at Apple support or is there a simpler answer?

Answer 1

A common Time Machine backup drive (with the root folder /Backups.backupdb/Computername/... only) itself is no bootable system. You always have to boot from Recovery Mode, Internet Recovery Mode or another bootable drive containing a bootable base or full OS X.

Booted to one of the mentioned systems allows you to restore the Time Machine backup to your main volume.

Additionally bootable systems require a GUID partition table and the volume has to be formatted with either HFS+ or HFS+ journaled. The latter is recommended/required for Time Machine backup drives. You may use an ExFAT volume but this needs additional configuration: you have to create a sparsebundle file, mount it and choose this one as backup volume.