Thanks Ludek,
Yes when I plugged in a fresh USB stick it registered in both Win and MM5 as H: drive so no inconsistency there, and yes the USB drive had previously been connected to MM, though a long time ago.
Thanks for the hint about naming the USB drive before introducing it to MM.
Cheers
Matt

Hi Matt,
this sounds that you originally connected the memory stick under the different drive letter (I:\) and there wasn't any volume label/name so it used 'I:\' as the default name for the device. Now it is connected as (H:\) so it looks confusing.

Tracking as https://www.ventismedia.com/mantis/view.php?id=19601

Feel free to rename the device name to prevent from the confusion until #19601 is fixed

When I plug a USB stick into my MM5 PC, the drive letter that is reported in Devices&Services (I:\ , being capital "i") is different from the drive letter reported in Win10 File Explorer and MM5 Folders (H:\). This is entirely repeatable and doesn't stop syncing from working properly; reporting a different drive letter just makes MM5 look bad.

My system has the following drive usage:
C: Win installation
D: Music partition
E: DVD drive
F: Wave files partition
G: External USB backup drive
H: External USB data stick sync target