Last week, I purchased a Silicon Power A55 SSD (1TB). I’m temporarily using the A55 to boot a 2015 MacBook Pro. The A55 boots up the MacBook Pro when connected via a USB to SATA connection, but the behavior is different when using a FireWire cable to SATA connection (Seagate GoFlex FW 800 cable).

With the A55 connected to GoFlex cable, the MacBook Pro boots part way up, but then a circle with a slash appears across the screen:

A55 connected via FireWire

Anyway, I filed a support ticket with Silicon Power to resolve this situation. It could be that this drive is not fully compatible with FireWire interfaces – stay tuned.