@neil @dascandy @linker3000 @fuzzySynths to be more precise (was running out of characters), it will boot with an external drive attached but there will be I/o errors constantly until the drive is disconnected and reconnected (physically). I think the start-up sequence doesnt give it enough power during initialisation and it doesn't recover (even with 2.5" ssd). Using a powered usb hub solved the problem. Later moving the drives out of the enclosures and onto a pi with sata hat was even better