Samsung T7 Shield suddenly very slow - other Samsung SSDs working fine?

I one T7 Shield 4TB and several non-Shield T7 and T5 drives - all 2TB. Until recently, they have all been working fine for both read and write on my Macbook Pro M1. I never benchmarked, but I was getting somewhere well in excess of 500MB/s. I used the T7 Shield for the first time in a few weeks today, I am getting decent read speeds still, but my write speeds have dropped very low - a current transfer is going at just 2MB/s. The read speeds are still good. I've tried using different cables and different USB ports on my Macbook. I've also used the same port/same cable with my other T7 and T5 drives and they are all working fine at full speed. The Shield T7 has over 2TB space free. Any idea what might be causing the slow speed? Anything I can do? Or is the drive broken? Thanks for any suggestions.