USB Card Readers
unRAID can boot from a USB card reader rather than a flash drive. The advantage of this approach is that typical flash drive fails are of the memory and not the rest of the device.
As such a USB card reader may be a better device choice as it would allow for flash memory replacement without the need to source a new license key.
However the choice of the correct USB card reader is crucial as most do not have an unique GUID and are therefore not suitable for unRAID.
This page captures know information.
WARNING! many flash memory and reader fakes exist that on examination are identical in appearance and function but do not typically have a unique GUID.
More reading in this discussion thread http://lime-technology.com/forum/index.php?topic=23008.0