This is short for "Slow" and is explained in the User Guide available to download from the A&H website
p.17:
Formatting USB Memory
ICE-16 will check the USB memory device each time it is connected.
Previously formatted devices will not be tested, and any recorded songs will be displayed numerically.
If the memory device is new and unformatted, ICE-16 will test it and display either the resolution mode Hi or Lo depending on the test result, or display 'SLo' if the memory device does not have sufficient speed to cope with the data rate with the memory format.
This may well be because the cluster size is set to 4, 8 or 16kbytes. It needs to be 32kbytes.
Formatting the USB memory on the ICE-16 will correct this.
So, if the initial test results in 'Slo' being displayed try formatting the memory to reset the cluster size and re-test the memory speed.