TheStoreJet 500​ ​by Transcend was previously introduced with a Thunderbolt connection for mirroring the backup of certain portable SSD with the emphasis on speed, but this time is released as​ ​Freecom mSSD, a portable SSD focusing on being small and lightweight.
A decade ago I was using this USB stick-sized SSD with 256GB of memory to quickly mirror my iTunes music library data used to DJ with as well as the mirror the Mac ‘Photo’ app library here in order to quickly backup after photo-shoots. Since synchronising between the ‘mothership’ and Mac Pro is such a hassle every time, I can’t help but use this to mirror each library.
Until, say a 1TB memory USB is released, I think I will keeping using this as a stop-gap.