Perhaps the MD600 is using SATA 1,5Gbs link speeds on the SATA controller.
Not all modern SSD’s are fully backwards compatible with this older standard, as far as I know.

#1 568.2mm, HD621, G91DH, 513, 511, DSP3100 , DSP33C , DSP33, 2x MJ Pro 50 MKI, M1500
#2 G68ADV, HD621, DSP3100, DSP33C, DSP33, 3x MJ Pro 50 MKII, Control 15, Vertex 2
#3 G61rsl, HD621, 596P, DSP7000, DSP5500HC, DSP33, 2x MJ Pro 50 MKII, M1500

Spare: , 2x 568.2mm, 562V.2, DSP33, Director, G55, M60.2, MS200,
Applied mods: NOS-SRx Filter by Mr. Apodizer