4.1GB? Is that > 4294967296 bytes? If so, it's likely that the filesystem you chose doesn't handle >4GB access properly, and as soon as the write occured past the border the RDB got nuked.
Use
check4gb to check it.
If the last partition does cross the 4GB border, you must edit it and make sure it doesn't cross it. Or alternatively use a filesystem that can handle TD64 or DirectSCSI (both of these are supported by the CS Mk-3 cybppc.device).
