I've changed the disk in my ND2901 from a 500GB HGST HDD to a WD Blue 1TB SSD.
Performance tests show that with the SSD I only get half the copy speed from the SD card as compared to the HDD before (15MB/s with SSD vs. 32MB/s with HDD) .
Is this normal behavior with SSD drives or is the particular to the WD Blue SSD?
That will be because of power problem.
Try connect AC power adapter and check the performance.
The copy speed of SD card is max 40MB/s independent of SSD or HDD.
You need to add external battery to use ND2901 with SSD.
As you can here, we does not recommend using SSD capacity larger than 512GB with ND2901 without external battery.