Use the right device name for your external ssd in these cmd’s:
hdparm -W /dev/sdb # show cache status
hdparm -W 0 /dev/sdb # disable cache
hdparm -W 1 /dev/sdb # enable cache
sdparm /dev/sdb|grep WCE # show cache status
sdparm --set WCE=0 /dev/sdb # disable cache
sdparm --set WCE=1 /dev/sdb # enable cache