Originally Posted by auspex
Interesting stuff. I bet it matters how you format the SD card too. The Kobo will automount FAT, EXT2/3/4 and probably NTFS (and loads of others), and some formats are bound to be faster than others.
FYI, You don't have to remove the external SD when performing a factory reset - it won't be touched, but it will be rescanned when the Kobo reboots.
The main reason for removing the external card is to speed up getting to the user interface after the factory reset. If everything looks okay, than I can insert the card and wait for it to be processed. Trying things like stuffing fonts into the upgrade has given some "interesting" results. As in Eric Frank Russell's "May you live in interesting times".
As for the card format, I've had bad experiences with the newer firmware and formats other than FAT32. They appeared to be tied to an attempt to create the overflow directory which the newer firmware creates on the external card. I've not tested this with 2.4.0.