A firmware update for the gy-hm850 is now on the JVC site. Version 0105-0034. I don't know what it fixes.

JVC Professional Customer Support - Firmware Upgrade (http://support.jvc.com/pro/firmwareupgrade/versionentry.jsp?modelId=57)

On the European JVC page, following is said about the V0105:

Maintenance update, improvement of BACKUP RECORDING on AVCHD format