PCIe SSD not recognized by BIOS

Hi! I have an Acer TravelMate x349-g2. I bought it with a 128Gb SSD (SATA600) and wanted to upgrade the storage. I bought a Plextor M9PeGN 512Gb PCIe SSD but the BIOS does not recognize it (it sees it, just doesn't recognize it). ("hdd identify data not found")

The interesting thing is that if I switch to legacy boot, the plextor boot screen appears (no such thing at UEFI boot mode), and there it recognizes the SSD, but than it gets stuck, and I can't even reach the BIOS (I have to detach the SSD to reach the BIOS).

I don't think that there is an interface incompatibility issue, because 1) in legacy the plextor boot screen appears, 2) this notebook is available with PCIe SSD-s as well, 3) the BIOS sees the SSD, it's just unable to recognize it.

I'm out of ideas, could anyone help me pls?
 
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