SD card problems

I bought a SanDisk Ultra 64GB microSDXC card for my Moto X4 phone so I
could have music on it. I have a SIIG USB 2.0 Multi Card Reader/Writer
that is several years old. The specs include: "Compliant with Universal
Serial Bus 2.0 Specification, Support up to 64GB memory card capacity,
Reads a variety of CF, xD, SD, SDHC, miniSD, microSD..."
One issue I found is when I first started saving files to the card
Windows 7 File Explorer reported that it was writing at 9.1MB/s, but I
notice that the speed steadily drops.
The next issue is that the card didn'preserve files beyond 7.16 GB,
twice it corrupted the files and Windows prompted me to Scan and Fix the
card when it was plugged into the card writer.
Also the write speed can vary now from about 3MB/s to 1 MB/s.
Does this sound like a bad card? It was only $15. Or could this be a bad
SIIG USB card writer?
I'm going to look around for a friend with a computer that has a built
in SD slot to see if I can rule out the SIIG.