Are you trying to boot LabVIEW Real-Time or Windows? Booting Windows is trickier. The rought requirements for booting from an ExpressCard device are:
1. The ExpressCard uses PCI Express connected to SATA-based storage. ExpressCard slots also provide USB connections, and some ExpressCard SSDs just use USB, which typically cannot be used for booting an OS.
2. (Windows only) The ExpressCard provides an Option ROM to enable booting from the device.
Finding cards that satisfy #1 is not too difficult. Requirement #2, finding ExpressCards that are natively bootable, is harder. This post here provides an overview of some of those issues.
For booting LabVIEW Real-Time, the native booting requirement does not exist because the support for booting LabVIEW Real-Time is directly built into the BIOS.